Cranelift: panic with appropriate error when pointer width mismatches the platform word size

I'm not sure what causes this, so I'll stick to a generic title..

### `.clif` Test Case

```
function u0:1(i64 vmctx) {
gv0 = vmctx
heap0 = static gv0

block0(v3: i64):
v1 = iconst.i32 0
v2 = iadd v1, v1
store little v1, v2
return
}
```

### Steps to Reproduce

```
⋊> cargo run --manifest-path=cranelift/Cargo.toml -- compile --target=x86_64 /tmp/test.clif
Finished dev [unoptimized + debuginfo] target(s) in 0.08s
Running `target/debug/clif-util compile --target=x86_64 /tmp/test.clif`
thread 'main' panicked at 'assertion failed: `(left == right)`
left: `types::I32`,
right: `types::I64`', cranelift/codegen/src/isa/x64/lower.rs:1289:9
stack backtrace:
0: rust_begin_unwind
[...]
3: core::panicking::assert_failed
4: cranelift_codegen::isa::x64::lower::lower_to_amode
at ./cranelift/codegen/src/isa/x64/lower.rs:1289:9
5: cranelift_codegen::isa::x64::lower::lower_insn_to_regs
at ./cranelift/codegen/src/isa/x64/lower.rs:4805:21
6: cranelift_codegen::isa::x64::lower::::lower
at ./cranelift/codegen/src/isa/x64/lower.rs:5755:9
7: cranelift_codegen::machinst::lower::Lower::lower_clif_block
at ./cranelift/codegen/src/machinst/lower.rs:736:17
8: cranelift_codegen::machinst::lower::Lower::lower
at ./cranelift/codegen/src/machinst/lower.rs:973:17
9: cranelift_codegen::machinst::compile::compile
at ./cranelift/codegen/src/machinst/compile.rs:30:9
10: cranelift_codegen::isa::x64::X64Backend::compile_vcode
at ./cranelift/codegen/src/isa/x64/mod.rs:50:9
11: ::compile_function
at ./cranelift/codegen/src/isa/x64/mod.rs:61:21
12: cranelift_codegen::context::Context::compile
at ./cranelift/codegen/src/context.rs:197:26
13: cranelift_codegen::context::Context::compile_and_emit
at ./cranelift/codegen/src/context.rs:137:20
14: clif_util::compile::handle_module
at ./cranelift/src/compile.rs:81:29
15: clif_util::compile::run
at ./cranelift/src/compile.rs:52:9
16: clif_util::main
at ./cranelift/src/clif-util.rs:130:33
```

A quick `git bisect` takes me to cb48ea406eea535b3108853b7b680d9aa21eb14b. That's probably not too useful though.