[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 +1,31 @@
+# RUN: llc -mtriple=aarch64 -mattr=+sme -run-pass=aarch64-post-coalescer-pass -o - %s | FileCheck %s
+
+---
+name:            foo
+body:             |
+  bb.0.entry:
+    RET_ReallyLR
+...
+---
+name:            bar
+machineFunctionInfo:
+  hasStreamingModeChanges: true
+body:             |
+  bb.0.entry:
+    ; CHECK-LABEL: name: bar
+    ; CHECK: 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 %0: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
+    ADJCALLSTACKDOWN 0, 0, implicit-def dead $sp, implicit $sp
+    %1:fpr64 = COALESCER_BARRIER_FPR64 undef %1
+    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
+    $d0 = COPY %1
+    BL @foo, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit killed $d0, implicit-def $sp
+    ADJCALLSTACKUP 0, 0, implicit-def dead $sp, implicit $sp
+    MSRpstatesvcrImm1 1, 0, csr_aarch64_smstartstop, implicit-def dead $nzcv, implicit $vg, implicit-def $vg, implicit-def $fpmr
+    RET_ReallyLR
+...