[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.mir

@@ -0,0 +1,16 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=aarch64 -mattr=+sme -run-pass=aarch64-post-coalescer-pass -o - %s | FileCheck %s
+
+---
+name:            foo
+machineFunctionInfo:
+  hasStreamingModeChanges: true
+body:             |
+  bb.0.entry:
+  ; CHECK-LABEL: name: foo
+  ; CHECK: $d0 = COPY undef %0:fpr64
+  ; CHECK-NEXT: FAKE_USE implicit $d0
+  %1:fpr64 = COALESCER_BARRIER_FPR64 undef %1
+  $d0 = COPY %1
+  FAKE_USE implicit $d0
+...