Initialize output parameter

[vitalybuka]

Summary

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

llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp

switch (Opc) {
     case Hexagon::A4_cmpbgtu:
     case Hexagon::A4_cmpheq:
     case Hexagon::A4_cmphgt:
     case Hexagon::A4_cmphgtu:
     case Hexagon::C4_cmpneq:
     case Hexagon::C4_cmplte:
     case Hexagon::C4_cmplteu:
       SrcReg2 = MI.getOperand(2).getReg();
+      Value = 0;
       return true;
 
     case Hexagon::C2_cmpeqi:
     case Hexagon::C2_cmpgtui:
     case Hexagon::C2_cmpgti:
     case Hexagon::C4_cmpneqi:
     case Hexagon::C4_cmplteui:
     case Hexagon::C4_cmpltei: