Cranelift: add a vreg limit check to correctly return an error on too-large inputs. (#4882)

Previously, Cranelift panicked (via a a panic in regalloc2) when the
virtual-register limit of 2M (2^21) was reached. This resulted in a
perplexing and unhelpful failure when the user provided a too-large
input (such as the Wasm module in #4865).

This PR adds an explicit check when allocating vregs that fails with a
"code too large" error when the limit is hit, producing output such as
(on the minimized testcase from #4865):

```
Error: failed to compile wasm function 3785 at offset 0xa3f3

Caused by:
    Compilation error: Code for function is too large
```

Fixes #4865.

cranelift/codegen/src/machinst/lower.rs

@@ -19,7 +19,7 @@ use crate::machinst::{
     LoweredBlock, MachLabel, Reg, SigSet, VCode, VCodeBuilder, VCodeConstant, VCodeConstantData,
     VCodeConstants, VCodeInst, ValueRegs, Writable,
 };
-use crate::{trace, CodegenResult};
+use crate::{trace, CodegenError, CodegenResult};
 use alloc::vec::Vec;
 use regalloc2::VReg;
 use smallvec::{smallvec, SmallVec};
@@ -323,6 +323,10 @@ fn alloc_vregs<I: VCodeInst>(
     let v = *next_vreg;
     let (regclasses, tys) = I::rc_for_type(ty)?;
     *next_vreg += regclasses.len();
+    if *next_vreg >= VReg::MAX {
+        return Err(CodegenError::CodeTooLarge);
+    }
+
     let regs: ValueRegs<Reg> = match regclasses {
         &[rc0] => ValueRegs::one(VReg::new(v, rc0).into()),
         &[rc0, rc1] => ValueRegs::two(VReg::new(v, rc0).into(), VReg::new(v + 1, rc1).into()),