[CodeGen] Skip some instructions that shouldn't affect shrink-wrapping

[thegameg]

Summary

r320606 checked for MI.isMetaInstruction which skips all DBG_VALUEs.

This also skips IMPLICIT_DEFs and other instructions that may def / read a register.

[MatzeB]

LGTM, thanks.

lib/CodeGen/ShrinkWrap.cpp

 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
 INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 INITIALIZE_PASS_END(ShrinkWrap, DEBUG_TYPE, "Shrink Wrap Pass", false, false)
 
 bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI,
                                  RegScavenger *RS) const {
-  // Ignore DBG_VALUE and other meta instructions that must not affect codegen.
-  if (MI.isMetaInstruction())
-    return false;
-
   if (MI.getOpcode() == FrameSetupOpcode ||
       MI.getOpcode() == FrameDestroyOpcode) {
     DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
     return true;
   }
   for (const MachineOperand &MO : MI.operands()) {
     bool UseOrDefCSR = false;
     if (MO.isReg()) {
+      // Ignore instructions like DBG_VALUE which don't read/def the register.
+      if (!MO.isDef() && !MO.readsReg())
+        continue;
       unsigned PhysReg = MO.getReg();
       if (!PhysReg)
         continue;
       assert(TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
              "Unallocated register?!");
       UseOrDefCSR = RCI.getLastCalleeSavedAlias(PhysReg);
     } else if (MO.isRegMask()) {
       // Check if this regmask clobbers any of the CSRs.
       for (unsigned Reg : getCurrentCSRs(RS)) {
         if (MO.clobbersPhysReg(Reg)) {
           UseOrDefCSR = true;
           break;
         }
       }
     }
-    if (UseOrDefCSR || MO.isFI()) {
+    // Skip FrameIndex operands in DBG_VALUE instructions.
+    if (UseOrDefCSR || (MO.isFI() && !MI.isDebugValue())) {
       DEBUG(dbgs() << "Use or define CSR(" << UseOrDefCSR << ") or FI("
                    << MO.isFI() << "): " << MI << '\n');
       return true;
     }
   }
   return false;
 }