[PEI] Don't zero out noreg operands

A tail call may have $noreg operands.

Fixes a crash.

Reviewed By: xgupta

Differential Revision: https://reviews.llvm.org/D156485

Author: aeubanks

llvm/lib/CodeGen/PrologEpilogInserter.cpp

@@ -1298,6 +1298,8 @@ void PEI::insertZeroCallUsedRegs(MachineFunction &MF) {
           continue;
 
         MCRegister Reg = MO.getReg();
+        if (!Reg)
+          continue;
 
         // This picks up sibling registers (e.q. %al -> %ah).
         for (MCRegUnit Unit : TRI.regunits(Reg))
@@ -1321,8 +1323,11 @@ void PEI::insertZeroCallUsedRegs(MachineFunction &MF) {
         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);
       }
     }

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

@@ -241,6 +241,20 @@ 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: