Cmm backend commons up variables with different types from different codeblocks if they share an unique.

Compiling this:

my_test ( W_ x1, W_ x2 ) {
    if (x1 == 0) {
        bits64 z;
        z = x2;
        return (z);
    } else {
        bits32 z;
        z = %lobits32(x2);
        return (z);
    }
}

Gives us this bogus cmm code:

==================== Parsed Cmm ====================
[my_test() { //  [R2, R1]
         { info_tbls: []
           stack_info: arg_space: 8
         }
     {offset
       c8: _c2::I64 = R2;
           _c1::I64 = R1;
           if (_c1::I64 == 0) goto c3; else goto c5;
       c3: _c6::I32 = _c2::I64;
           R1 = %MO_UU_Conv_W32_W64(_c6::I32);
           call (P64[(old + 8)])(R1) args: 8, res: 0, upd: 8;
       c5: _c6::I32 = %MO_UU_Conv_W64_W32(_c2::I64);
           R1 = %MO_UU_Conv_W32_W64(_c6::I32);
           call (P64[(old + 8)])(R1) args: 8, res: 0, upd: 8;
     }
 }]

Which triggers lint because we end up giving both z variables the same type:

Cmm lint error:
  in basic block c3
    in assignment:
      _c6::I32 = _c2::I64;
      Reg ty: I32
      Rhs ty: I64