[PowerPC] Remove assertion "Shouldn't overwrite a register before it is killed"

[Yi-Hong.Lyu]

Summary

The assertion is everzealous and fail tests like:

renamable $x3 = LI8 0
STD renamable $x3, 16, $x1
renamable $x3 = LI8 0

Remove the assertion since killed flag of $x3 is not mandentory.

[stefanp]

Thank you for fixing this.
LGTM.

llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp

bool removeRedundantLIs(MachineBasicBlock &MBB,
             DeadOrKillToUnset = &AfterBBI->getOperand(KillIdx);
             LLVM_DEBUG(dbgs()
                        << " Kill flag of " << *DeadOrKillToUnset << " from "
                        << *AfterBBI << " is a unsetting candidate\n");
           }
 
           if (!AfterBBI->modifiesRegister(Reg, TRI))
             continue;
-          assert(DeadOrKillToUnset &&
-                 "Shouldn't overwrite a register before it is killed");
           // Finish scanning because Reg is overwritten by a non-load
           // instruction.
           if (AfterBBI->getOpcode() != Opc)
             break;
           assert(AfterBBI->getOperand(0).isReg() &&
                  "Expected a register for the first operand");
           // Finish scanning because Reg is overwritten by a relocation or a
           // different value.
           if (!AfterBBI->getOperand(1).isImm() ||
               AfterBBI->getOperand(1).getImm() != Imm)
             break;
 
           // It loads same immediate value to the same Reg, which is redundant.
           // We would unset kill flag in previous Reg usage to extend live range
           // of Reg first, then remove the redundancy.
-          LLVM_DEBUG(dbgs() << " Unset dead/kill flag of " << *DeadOrKillToUnset
-                            << " from " << *DeadOrKillToUnset->getParent());
+          if (DeadOrKillToUnset) {
+            LLVM_DEBUG(dbgs()
+                       << " Unset dead/kill flag of " << *DeadOrKillToUnset
             if (DeadOrKillToUnset->isDef())
               DeadOrKillToUnset->setIsDead(false);
             else
               DeadOrKillToUnset->setIsKill(false);
+          }
           DeadOrKillToUnset =
               AfterBBI->findRegisterDefOperand(Reg, true, true, TRI);
           if (DeadOrKillToUnset)
             LLVM_DEBUG(dbgs()
                        << " Dead flag of " << *DeadOrKillToUnset << " from "
                        << *AfterBBI << " is a unsetting candidate\n");
           InstrsToErase.insert(&*AfterBBI);
           LLVM_DEBUG(dbgs() << " Remove redundant load immediate: ";

llvm/test/CodeGen/PowerPC/remove-redundant-load-imm.mir

bb.0.entry:
     renamable $x4 = ADDI8 $x1, -128
     dead renamable $r3 = LI 128
     renamable $x5 = ADDI8 $x1, -128
     renamable $r3 = LI 128
     STW killed renamable $r3, 16, $x4
     BLR8 implicit $lr8, implicit $rm
 
 ...
+---
+name:            overwrite_reg_before_killed
+alignment:       16
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    liveins: $x1
+
+    ; CHECK-LABEL: name: overwrite_reg_before_killed
+    ; CHECK: liveins: $x1
+    ; CHECK: renamable $x3 = LI8 0
+    ; CHECK: STD renamable $x3, 16, $x1
+    ; CHECK: STD killed renamable $x3, 8, $x1
+    ; CHECK: BLR8 implicit $lr8, implicit $rm
+    renamable $x3 = LI8 0
+    STD renamable $x3, 16, $x1
+    renamable $x3 = LI8 0
+    STD killed renamable $x3, 8, $x1
+    BLR8 implicit $lr8, implicit $rm
+
+...