[AArch64PostCoalescer] Preserve COALESCER_BARRIER for active register uses

If the destination register still has active uses, removing
COALESCER_BARRIER is unsafe.

Author: XiaShark

llvm/lib/Target/AArch64/AArch64PostCoalescerPass.cpp

@@ -72,6 +72,8 @@ bool AArch64PostCoalescer::runOnMachineFunction(MachineFunction &MF) {
       case AArch64::COALESCER_BARRIER_FPR128: {
         Register Src = MI.getOperand(1).getReg();
         Register Dst = MI.getOperand(0).getReg();
+        if (MRI->hasOneDef(Dst) && !MRI->use_empty(Dst))
+          continue;
         if (Src != Dst)
           MRI->replaceRegWith(Dst, Src);
 

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

@@ -0,0 +1,14 @@
+; RUN: llc -mtriple=aarch64 -mattr=+sme -stop-ater=aarch64-post-coalescer-pass < %s | FileCheck %s
+
+; CHECK-LABEL: name: bar
+; CHECK: [[dst:%[0-9]+]]:fpr64 = COALESCER_BARRIER_FPR64 undef [[src:%[0-9]+]]
+
+target triple = "aarch64"
+
+declare void @foo(double) "aarch64_pstate_sm_enabled"
+
+define dso_local void @bar(double noundef %a) local_unnamed_addr {
+entry:
+  tail call void @foo(double poison) "aarch64_pstate_sm_enabled"
+  ret void
+}