Initialize output parameter

Or code like this have UB passing uninitialized CmpValue:

```
  int64_t CmpMask, CmpValue;
  if (!TII->analyzeCompare(MI, SrcReg, SrcReg2, CmpMask, CmpValue))
    return false;
  if (TII->optimizeCompareInstr(MI, SrcReg, SrcReg2, CmpMask, CmpValue, MRI)) {
```

Detected by msan with:
-Xclang -enable-noundef-analysis -mllvm -msan-eager-checks=1

Differential Revision: https://reviews.llvm.org/D116831