[AArch64PostCoalescer] Propagate undef flag after replacing

When replacing the register, if the source register has undef flag, we
should propagate the flag to all uses of the destination register.

Author: XiaShark

llvm/lib/Target/AArch64/AArch64PostCoalescerPass.cpp

@@ -75,6 +75,10 @@ bool AArch64PostCoalescer::runOnMachineFunction(MachineFunction &MF) {
         if (Src != Dst)
           MRI->replaceRegWith(Dst, Src);
 
+        if (MI.getOperand(1).isUndef())
+          for (MachineOperand &MO : MRI->use_operands(Dst))
+            MO.setIsUndef();
+
         // MI must be erased from the basic block before recalculating the live
         // interval.
         LIS->RemoveMachineInstrFromMaps(MI);

llvm/test/CodeGen/AArch64/aarch64-post-coalescer.ll

@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=aarch64 -mattr=+sme -stop-after=aarch64-post-coalescer-pass < %s | FileCheck %s
+
+target triple = "aarch64"
+
+declare void @foo(double) "aarch64_pstate_sm_enabled"
+
+define dso_local void @bar(double %a) local_unnamed_addr {
+  ; CHECK-LABEL: name: bar
+  ; CHECK: bb.0.entry:
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN 0, 0, implicit-def dead $sp, implicit $sp
+  ; CHECK-NEXT:   MSRpstatesvcrImm1 1, 1, csr_aarch64_smstartstop, implicit-def dead $nzcv, implicit-def $sp, implicit-def dead $d0, implicit $vg, implicit-def $vg, implicit-def $fpmr
+  ; CHECK-NEXT:   $d0 = COPY undef %1:fpr64
+  ; CHECK-NEXT:   BL @foo, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit killed $d0, implicit-def $sp
+  ; CHECK-NEXT:   ADJCALLSTACKUP 0, 0, implicit-def dead $sp, implicit $sp
+  ; CHECK-NEXT:   MSRpstatesvcrImm1 1, 0, csr_aarch64_smstartstop, implicit-def dead $nzcv, implicit $vg, implicit-def $vg, implicit-def $fpmr
+  ; CHECK-NEXT:   RET_ReallyLR
+entry:
+  tail call void @foo(double poison) "aarch64_pstate_sm_enabled"
+  ret void
+}