Segfault when trying to manually build a dynamo-like function

I tried to disassemble and understand the process of how dynamos are working, and reasoned about what would be equivalent the following dynamo:

```
IRTools.@dynamo function test_dynamo(a...)
ir = empty(IRTools.IR(a...))
push!(ir, IRTools.xcall(Main, :println, "hi"))
IRTools.return!(ir, nothing)
return ir
end
```

Naively substituting the code pieces in [dynamo.jl](https://github.com/MikeInnes/IRTools.jl/blob/master/src/reflection/dynamo.jl), I thought the following should work:

```
@generated function test1(f, args...)
ir = IRTools.empty(IRTools.IR(f, args...))
push!(ir, IRTools.xcall(Main, :println, "hi"))
IRTools.return!(ir, nothing)

m = ir.meta::IRTools.Meta
ir = IRTools.varargs!(m, ir)
IRTools.argnames!(m, :args)
_self = IRTools.splicearg!(m, ir, Symbol("#self#"))
IRTools.prewalk!(x -> x === IRTools.self ? _self : x, ir)
@show result = IRTools.update!(m.code, ir)
return result
end
```

But instead, it crashes:

```
julia> test1(identity, 1)
result = IRTools.update!(m.code, ir) = CodeInfo(
1 ─ (Main.println)("hi")
│ %2 = nothing
└── return %2
)
Internal error: encountered unexpected error in runtime:
BoundsError(a=Array{Core.Compiler.SlotInfo, (2,)}[
Core.Compiler.SlotInfo(defs=Array{Int64, (0,)}[], uses=Array{Int64, (0,)}[], any_newvar=false),
Core.Compiler.SlotInfo(defs=Array{Int64, (0,)}[], uses=Array{Int64, (0,)}[], any_newvar=false)], i=(Core.Compiler.UnitRange{Int64}(start=1, stop=3),))
rec_backtrace at /buildworker/worker/package_linux64/build/src/stackwalk.c:94
record_backtrace at /buildworker/worker/package_linux64/build/src/task.c:246
jl_throw at /buildworker/worker/package_linux64/build/src/task.c:577
throw_boundserror at ./abstractarray.jl:484
checkbounds at ./abstractarray.jl:449 [inlined]
getindex at ./array.jl:737 [inlined]
scan_slot_def_use at ./compiler/ssair/slot2ssa.jl:51
just_construct_ssa at ./compiler/ssair/driver.jl:103
run_passes at ./compiler/ssair/driver.jl:114
optimize at ./compiler/optimize.jl:162
typeinf at ./compiler/typeinfer.jl:35
typeinf_ext at ./compiler/typeinfer.jl:567
typeinf_ext at ./compiler/typeinfer.jl:604
jfptr_typeinf_ext_1 at /usr/local/julia-1.0.0/lib/julia/sys.so (unknown line)
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1536 [inlined]
jl_apply_with_saved_exception_state at /buildworker/worker/package_linux64/build/src/rtutils.c:257
jl_type_infer at /buildworker/worker/package_linux64/build/src/gf.c:275
jl_compile_method_internal at /buildworker/worker/package_linux64/build/src/gf.c:1784 [inlined]
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1828
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:324
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:428
eval_stmt_value at /buildworker/worker/package_linux64/build/src/interpreter.c:363 [inlined]
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:686
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:799
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f69e453496f)
unknown function (ip: 0xffffffffffffffff)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:808
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:787
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/builtins.c:622
eval at ./boot.jl:319
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
eval_user_input at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/REPL/src/REPL.jl:85
run_backend at /home/philipp/.julia/packages/Revise/agmgx/src/Revise.jl:949
#75 at ./task.jl:259
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1829
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1536 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:268
unknown function (ip: 0xffffffffffffffff)
result = IRTools.update!(m.code, ir) = CodeInfo(
1 ─ (Main.println)("hi")
│ %2 = nothing
└── return %2
)

signal (11): Segmentation fault
in expression starting at no file:0
cast_or_null at /buildworker/worker/package_linux64/build/usr/include/llvm/Support/Casting.h:298 [inlined]
emit_function at /buildworker/worker/package_linux64/build/src/codegen.cpp:6552
jl_compile_linfo at /buildworker/worker/package_linux64/build/src/codegen.cpp:1159
jl_generate_fptr at /buildworker/worker/package_linux64/build/src/codegen.cpp:1394
jl_compile_method_internal at /buildworker/worker/package_linux64/build/src/gf.c:1821 [inlined]
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1828
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:324
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:428
eval_stmt_value at /buildworker/worker/package_linux64/build/src/interpreter.c:363 [inlined]
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:686
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:799
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f69e453496f)
unknown function (ip: 0xffffffffffffffff)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:808
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:787
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/builtins.c:622
eval at ./boot.jl:319
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
eval_user_input at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/REPL/src/REPL.jl:85
run_backend at /home/philipp/.julia/packages/Revise/agmgx/src/Revise.jl:949
#75 at ./task.jl:259
jl_fptr_trampoline at /buildworker/worker/package_linux64/build/src/gf.c:1829
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1536 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:268
unknown function (ip: 0xffffffffffffffff)
Allocations: 21604532 (Pool: 21601220; Big: 3312); GC: 48
[1] 26428 segmentation fault (core dumped) julia
```

No idea if that's a legitimate use case that should have worked. What I actually wanted to achieve was to get a dynamo that adds a new argument to the given method.