Cranelift: support casting reference-typed values to/from integers

For the below `.clif` code Cranelift appears to pass unsolvable input to regalloc. This was previously investigated as [an issue with regalloc](https://github.com/bytecodealliance/regalloc.rs/issues/124), but after some investigation @cfallin suggested moving it here.

This has been observed to cause failures when using regalloc and the upcoming regalloc2.

### `.clif` Test Case

```
function u0:0() -> r64 system_v {
sig0 = (i64) -> r64 system_v
sig1 = (r64, r64, r64) -> r64 system_v
sig2 = (i64) -> r64 system_v
sig3 = (i64) -> r64 system_v
sig4 = (r64, r64, r64) -> r64 system_v
sig5 = (r64, r64, r64) -> r64 system_v
fn0 = colocated u0:2 sig0
fn1 = colocated u0:5 sig1
fn2 = colocated u0:2 sig2
fn3 = colocated u0:2 sig3

block0:
v0 = iconst.i64 8
v1 = call fn0(v0)
v2 = func_addr.r64 fn1
store v2, v1
v3 = raw_bitcast.i64 v1
v4 = bor_imm v3, 6
v5 = raw_bitcast.r64 v4
v6 = raw_bitcast.i64 v5
v7 = band_imm v6, -8
v8 = raw_bitcast.r64 v7
v9 = load.r64 v8
v10 = iconst.i64 1
v11 = raw_bitcast.r64 v10
v12 = iconst.i64 8
v13 = call fn2(v12)
v14 = raw_bitcast.i64 v5
v15 = band_imm v14, -8
v16 = raw_bitcast.r64 v15
v17 = load.r64 v16
v18 = iconst.i64 1
v19 = raw_bitcast.r64 v18
v20 = iconst.i64 8
v21 = call fn3(v20)
v22 = iconst.i64 8
v23 = raw_bitcast.r64 v22
store v23, v21
v24 = call_indirect sig4, v17(v16, v19, v21)
store v24, v13
v25 = call_indirect sig5, v9(v8, v11, v13)
return v25
}
```

### Steps to Reproduce

1. Place `.clif` test case above in a file `fn.clif`
2. Run `cargo run -p cranelift-tools -- compile --target x86_64 -D fn.clif` and observe a panic.
```
thread 'main' panicked at 'assertion failed: vlr_env[cand_vlrix].is_ref == is_ref', /Users/zekemedley/.cargo/registry/src/github.com-1ecc6299db9ec823/regalloc-0.0.31/src/bt_spillslot_allocator.rs:298:13
```

### Reproduction with regalloc2

1. Place `.clif` test case above in a file `fn.clif`
2. Follow the insutructions [here](https://github.com/bytecodealliance/regalloc.rs/issues/124#issuecomment-871731448) for compiling Cranelift with regalloc2.
3. Run `cargo run -p cranelift-tools -- compile --target x86_64 -D fn.clif` and observe a panic.
```
thread 'main' panicked at 'Minimal bundle with conflict!', /Users/zekemedley/Desktop/projects/lust/regalloc-bug/regalloc2/src/ion/process.rs:763:13
```