[PEI] Don't zero out noreg operands

[aeubanks]

Summary

A tail call may have $noreg operands.

Fixes a crash.

[arsenm]

llvm/lib/CodeGen/PrologEpilogInserter.cpp

1330

might as well remove the & if you're touching this

[aeubanks]

no &

[xgupta]

LGTM, Thanks!

llvm/lib/CodeGen/PrologEpilogInserter.cpp

for (const MachineInstr &MI : MBB.terminators()) {
       if (!MI.isReturn())
         continue;
 
       for (const auto &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
 
         MCRegister Reg = MO.getReg();
+        if (!Reg)
+          continue;
 
         // This picks up sibling registers (e.q. %al -> %ah).
         for (MCRegUnit Unit : TRI.regunits(Reg))
           RegsToZero.reset(Unit);
 
         for (MCPhysReg SReg : TRI.sub_and_superregs_inclusive(Reg))
           RegsToZero.reset(SReg);
       }
     }
 
   // Don't need to clear registers that are used/clobbered by terminating
   // instructions.
   for (const MachineBasicBlock &MBB : MF) {
     if (!MBB.isReturnBlock())
       continue;
 
     MachineBasicBlock::const_iterator MBBI = MBB.getFirstTerminator();
     for (MachineBasicBlock::const_iterator I = MBBI, E = MBB.end(); I != E;
          ++I) {
       for (const MachineOperand &MO : I->operands()) {
         if (!MO.isReg())
           continue;
 
-        for (const MCPhysReg &Reg :
-             TRI.sub_and_superregs_inclusive(MO.getReg()))
+        MCRegister Reg = MO.getReg();
+        if (!Reg)
+          continue;
+
+        for (const MCPhysReg Reg : TRI.sub_and_superregs_inclusive(Reg))
           RegsToZero.reset(Reg);
       }
     }
   }
 
   // Don't clear registers that must be preserved.
   for (const MCPhysReg *CSRegs = TRI.getCalleeSavedRegs(&MF);
        MCPhysReg CSReg = *CSRegs; ++CSRegs)

llvm/test/CodeGen/X86/zero-call-used-regs.ll

 ; X86-64-NEXT:    xorps %xmm14, %xmm14
 ; X86-64-NEXT:    xorps %xmm15, %xmm15
 ; X86-64-NEXT:    retq
 
 entry:
   ret i32 %x
 }
 
+define dso_local void @tailcall(ptr %p) local_unnamed_addr #0 "zero-call-used-regs"="used-gpr" {
+; I386-LABEL: tailcall:
+; I386:       # %bb.0:
+; I386-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; I386-NEXT:    jmpl *(%eax) # TAILCALL
+;
+; X86-64-LABEL: tailcall:
+; X86-64:       # %bb.0:
+; X86-64-NEXT:    jmpq *(%rdi) # TAILCALL
+  %c = load ptr, ptr %p
+  tail call void %c()
+  ret void
+}
+
 ; Don't emit zeroing registers in "main" function.
 define dso_local i32 @main() local_unnamed_addr #1 {
 ; I386-LABEL: main:
 ; I386:       # %bb.0: # %entry
 ; I386-NEXT:    xorl %eax, %eax
 ; I386-NEXT:    retl
 ;
 ; X86-64-LABEL: main: