[AMDGPU] Fix crash in phi-elimination hook.

[hliao]

Summary

• Pre-check in case there's just a single PHI insn.

[rampitec]

LGTM

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

if (I->modifiesRegister(AMDGPU::EXEC, TRI))
       return true;
   }
 }
 
 MachineInstr *SIInstrInfo::createPHIDestinationCopy(
     MachineBasicBlock &MBB, MachineBasicBlock::iterator LastPHIIt,
     const DebugLoc &DL, Register Src, Register Dst) const {
   auto Cur = MBB.begin();
-  do {
+  while (Cur != MBB.end()) {
     if (!Cur->isPHI() && Cur->readsRegister(Dst))
       return BuildMI(MBB, Cur, DL, get(TargetOpcode::COPY), Dst).addReg(Src);
     ++Cur;
-  } while (Cur != MBB.end() && Cur != LastPHIIt);
+    if (Cur == LastPHIIt)
+      break;
+  }
 
   return TargetInstrInfo::createPHIDestinationCopy(MBB, LastPHIIt, DL, Src,
                                                    Dst);
 }
 
 MachineInstr *SIInstrInfo::createPHISourceCopy(
     MachineBasicBlock &MBB, MachineBasicBlock::iterator InsPt,
     const DebugLoc &DL, Register Src, Register SrcSubReg, Register Dst) const {

llvm/test/CodeGen/AMDGPU/phi-elimination-assertion.mir

bb.2:
     %2:sreg_32_xm0 = S_MOV_B32 254
 
   bb.3:
     %4:sreg_32_xm0 = PHI %2, %bb.2, %1, %bb.1
     dead %3:sreg_32_xm0 = PHI undef %2, %bb.2, undef %1, %bb.1
     S_NOP 0, implicit %4
 ...
 
+# The following test crashes in phi-elimination hooks.
+#
 # CHECK-LABEL: name:            bar
 # CHECK:   bb.3:
 # CHECK-NEXT:     dead %3:sreg_32_xm0 = IMPLICIT_DEF
 # CHECK-NEXT:     %2:sreg_32_xm0 = COPY killed %4
 # CHECK-NEXT:     S_NOP 0, implicit killed %2
+
+---
+name:            bax
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    S_CBRANCH_SCC0 %bb.2, implicit undef $scc
+
+  bb.1:
+    %1:sreg_32_xm0 = S_MOV_B32 255
+    S_BRANCH %bb.3
+
+  bb.2:
+    %2:sreg_32_xm0 = S_MOV_B32 254
+
+  bb.3:
+    %3:sreg_32_xm0 = PHI %2, %bb.2, %1, %bb.1
+...
+
+# CHECK-LABEL: name:            bax
+# CHECK:   bb.3:
+# CHECK-NEXT:     %2:sreg_32_xm0 = COPY killed %3