Calling tagToEnum#, and then branching on the result produces bad code.

It results in Cmm like this:

           unwind Sp = Just Sp;
//The "call" to tagToEnum#
           _s3S8::P64 = I64[(_s3S5::I64 << 3) + Main.ItemType_closure_tbl];
//The usual "check if evaluated - if not evaluate prolog
//where we spill all the live variables.
           I64[Sp - 184] = c5Qc;
           R1 = _s3S8::P64;
           P64[Sp - 176] = P64[_s3S4::P64 + 63];
           P64[Sp - 168] = P64[_s3S4::P64 + 47];
           P64[Sp - 160] = P64[_s3S4::P64 + 71];
           P64[Sp - 152] = P64[_s3S4::P64 + 7];
           P64[Sp - 144] = P64[_s3S4::P64 + 39];
           P64[Sp - 136] = P64[_s3S4::P64 + 23];
           P64[Sp - 128] = P64[_s3S4::P64 + 31];
           I64[Sp - 120] = I64[_s3S4::P64 + 151];
           I64[Sp - 112] = I64[_s3S4::P64 + 143];
           P64[Sp - 104] = P64[_s3S4::P64 + 15];
           P64[Sp - 96] = P64[_s3S4::P64 + 79];
           P64[Sp - 88] = P64[_s3S4::P64 + 135];
           P64[Sp - 80] = P64[_s3S4::P64 + 127];
           P64[Sp - 72] = P64[_s3S4::P64 + 119];
           P64[Sp - 64] = P64[_s3S4::P64 + 55];
           P64[Sp - 56] = P64[_s3S4::P64 + 111];
           P64[Sp - 48] = P64[_s3S4::P64 + 103];
           P64[Sp - 40] = P64[_s3S4::P64 + 87];
           P64[Sp - 32] = P64[_s3S4::P64 + 95];
           P64[Sp - 24] = _s3S4::P64;
           I64[Sp - 16] = _s3S5::I64;
           P64[Sp - 8] = _s3S8::P64;
           Sp = Sp - 184;
           unwind Sp = Just Sp + 184;
//Check tag
           if (R1 & 7 != 0) goto c5Qc; else goto c5Uv;
       c5Uv:
           unwind Sp = Just Sp + 184;
//Enter closure if untaged
           call (I64[R1])(R1) returns to c5Qc, args: 8, res: 8, upd: 8;
       c5Qc:
//Having ensured the closure is tagged we can finally branch on the result.
           unwind Sp = Just Sp + 184;
           _c5Va::P64 = R1 & 7;
           if (_c5Va::P64 < 3) goto u5Ve; else goto u5Vf;

I noticed this while working on tag inference. Maybe we could be altogether smarter here. After all tagToEnum# will always produce a tagged value.

#16970 (closed) is one way how we could avoid this.