Add a coercion/cast operator to CMM

While trying to implement casts between floating-point and integral types #4092 (closed)) @rwbarton suggested the following CMM:

stg_wordToFloatzh(W_ w)
{
    D_ d;
    STK_CHK_P_LL(WDS(1), stg_wordToFloatzh, w);
    Sp(-1) = w;
    d = Sp(-1);
    return (d);
}

This code works outside of GHC, but when I try to add it as rts/CastFloatWord.cmm the CMM linter complains with:

Cmm lint error:
  in basic block c5
    in assignment: 
      _c2::F64 = I64[Sp + -1 * 8];
      Reg ty: F64
      Rhs ty: I64
Program was:
  {offset
    c6: // global
        _c1::I64 = R1;
        //tick src<rts/CastFloatWord.cmm:(7,1)-(13,1)>
        if (Sp - 1 * 8 < SpLim) goto c3; else goto c5;
    c3: // global
        //tick src<rts/CastFloatWord.cmm:9:35-100>
        R1 = _c1::I64;
        R2 = stg_wordToFloatzh;
        call stg_gc_prim_p_ll(R1, R2) args: 8, res: 0, upd: 8;
    c5: // global
        I64[Sp + -1 * 8] = _c1::I64;
        _c2::F64 = I64[Sp + -1 * 8];
        D1 = _c2::F64;
        call (P64[(old + 8)])(D1) args: 8, res: 0, upd: 8;
  }

For hand written CMM code like this, it would be nice to have a cast/coercion to allow this code to pass the linter.

Looking for suggestions as to the syntax. I suspect that it would be best if the destination type was made explicit in the cast operation.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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