Cranelift crash in riscv64 isle

### Test Case

Reduce from libsodium `kx` using `wasm-tools shrink`

```
(module
(type (;0;) (func (param i32 i32)))
(type (;1;) (func (param i32 i32 i32)))
(memory (;0;) 2)
(func (;0;) (type 1) (param i32 i32 i32))
(func (;1;) (type 0) (param i32 i32)
(local i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32)
loop ;; label = @1
i32.const 0
local.get 1
i32.store8
local.get 3
br_if 0 (;@1;)
end
i32.const 1
local.get 0
i32.store8
local.get 0
local.get 0
i32.store8
loop ;; label = @1
local.get 0
local.get 0
i32.store8
local.get 1
local.get 4
i32.store8
i32.const 0
br_if 0 (;@1;)
end
loop ;; label = @1
i32.const 0
local.get 1
i32.store8
local.get 1
br_if 0 (;@1;)
end
i32.const 0
i32.const 1
i32.store8 offset=351
i32.const 1
i32.const 0
i32.const 40
memory.fill
local.get 6
i32.const 1
i32.store
local.get 6
local.get 6
local.get 6
memory.fill
local.get 6
i32.const 1
i32.store
local.get 6
i32.const 0
i32.const 0
memory.fill
loop ;; label = @1
local.get 1
local.get 1
local.get 1
call 8
local.get 5
local.get 5
local.get 5
call 7
local.get 6
i32.const 1
call 5
i32.const 0
br_if 0 (;@1;)
end
i32.const 1
local.get 6
i32.const 40
memory.copy
local.get 5
local.get 5
i32.const 1
memory.copy
local.get 5
local.get 5
i32.const 0
memory.copy
local.get 1
local.get 1
call 6
local.get 1
local.get 4
call 4
local.get 1
local.get 1
call 6
local.get 1
local.get 4
call 4
i32.const 0
i32.const 1
call 6
local.get 1
local.get 4
call 4
i32.const 1
i32.const 0
call 6
local.get 6
local.get 4
call 5
loop ;; label = @1
local.get 1
local.get 1
local.get 1
call 8
local.get 4
local.get 4
local.get 4
call 7
local.get 6
local.get 7
call 5
local.get 1
br_if 0 (;@1;)
end
nop
local.get 2
local.get 2
i32.store offset=292
i32.const 0
local.get 18
i32.store offset=288
local.get 2
local.get 2
i32.store offset=284
local.get 2
local.get 14
i32.store offset=280
i32.const 0
local.get 12
i32.store offset=276
i32.const 0
local.get 10
i32.store offset=272
local.get 8
i32.const 1
i32.store offset=268
i32.const 0
i32.const 1
i32.store offset=264
local.get 2
local.get 2
i32.store offset=260
local.get 1
local.get 1
i32.store offset=256
local.get 2
i32.const 1
i32.store offset=244
local.get 2
i32.const 0
i32.store offset=240
local.get 2
i32.const 0
i32.store offset=236
i32.const 0
i32.const 0
i32.store offset=232
local.get 2
local.get 12
i32.store offset=228
local.get 2
local.get 10
i32.store offset=224
local.get 2
local.get 8
i32.store offset=220
local.get 2
local.get 4
i32.store offset=216
i32.const 0
local.get 5
i32.store offset=212
local.get 1
local.get 1
i32.store offset=208
local.get 2
local.get 1
call 3
local.get 1
local.get 1
local.get 1
call 0
local.get 0
local.get 2
call 2
)
(func (;2;) (type 0) (param i32 i32))
(func (;3;) (type 0) (param i32 i32))
(func (;4;) (type 0) (param i32 i32))
(func (;5;) (type 0) (param i32 i32))
(func (;6;) (type 0) (param i32 i32))
(func (;7;) (type 1) (param i32 i32 i32))
(func (;8;) (type 1) (param i32 i32 i32))
)
```

### Steps to Reproduce

* `wasmtime compile --target riscv64gc-unknown-linux-gnu kx.shrunken.wasm`

### Expected Results

It should compile like wasmtime 45

### Actual Results

```
thread '' (663301) panicked at /home/han/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cranelift-codegen-0.134.2/src/isa/riscv64/inst/args.rs:1271:18:
internal error: entered unreachable code
stack backtrace:
0: __rustc::rust_begin_unwind
1: core::panicking::panic_fmt
2: core::panicking::panic
3: cranelift_codegen::isa::riscv64::lower::isle::generated_code::constructor_gen_store::>
4: cranelift_codegen::isa::riscv64::lower::isle::generated_code::constructor_lower::>
5: ::lower
6: cranelift_codegen::machinst::compile::compile::
7: ::compile_function
8: ::compile_stencil
9: ::compile
10: ::finish_with_info
11: ::finish_compiling
12: <&::run_maybe_parallel core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>, wasmtime::compile::CompileOutput, wasmtime_internal_core::error::error::Error, ::compile::{closure#0}>::{closure#0} as core::ops::function::FnMut<(alloc::boxed::Box core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>,)>>::call_mut
13: > as rayon::iter::plumbing::Folder>>::consume_iter:: core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>>, &::run_maybe_parallel core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>, wasmtime::compile::CompileOutput, wasmtime_internal_core::error::error::Error, ::compile::{closure#0}>::{closure#0}>>
14: rayon::iter::plumbing::bridge_producer_consumer::helper:: core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>>, rayon::iter::map::MapConsumer>, ::run_maybe_parallel core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>, wasmtime::compile::CompileOutput, wasmtime_internal_core::error::error::Error, ::compile::{closure#0}>::{closure#0}>>
15: >, rayon::iter::plumbing::bridge_producer_consumer::helper core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>>, rayon::iter::map::MapConsumer>, ::run_maybe_parallel core::ops::function::FnOnce<(&'a dyn wasmtime_environ::compile::Compiler + 'a,), Output = core::result::Result> + core::marker::Send>, wasmtime::compile::CompileOutput, wasmtime_internal_core::error::error::Error, ::compile::{closure#0}>::{closure#0}>>::{closure#1}>::{closure#0}, rayon::iter::collect::consumer::CollectResult>> as rayon_core::job::Job>::execute
16: ::wait_until_cold
17: ::run
```

The type entering unreachable branch is `types::I8X16`.

### Versions and Environment

Wasmtime version or commit: `wasmtime 47.0.2` or `46.0.1`

Operating system: Gentoo Linux glibc-2.43-r2 linux 7.14

Architecture: host: amd64, target: riscv64gc

Reproduce the failure with `wasmtime compile --target riscv64gc-unknown-linux-gnu kx.shrunken.wasm`, then inspect the RISC-V ISLE lowering path and `isa/riscv64/inst/args.rs` around line 1271, where `types::I8X16` enters unreachable code. Done means the supplied module compiles for the riscv64gc target without the panic.

Written by the indexing model from the issue text.