[llvm] [AArch64][GlobalISel] Add combine for build_vector(unmerge, unmerge, undef, undef) (PR #165539)

[Matt Arsenault via llvm-commits]

================
@@ -133,6 +134,123 @@ bool isZeroExtended(Register R, MachineRegisterInfo &MRI) {
   return MRI.getVRegDef(R)->getOpcode() == TargetOpcode::G_ZEXT;
 }
 
+// This pattern aims to match the following shape to avoid extra mov
+// instructions
+// G_BUILD_VECTOR(
+//   G_UNMERGE_VALUES(src, 0)
+//   G_UNMERGE_VALUES(src, 1)
+//   G_IMPLICIT_DEF
+//   G_IMPLICIT_DEF
+// )
+// ->
+// G_CONCAT_VECTORS(
+//   src,
+//   undef
+// )
+bool matchCombineBuildUnmerge(MachineInstr &MI, MachineRegisterInfo &MRI,
+                              Register &UnmergeSrc) {
+  assert(MI.getOpcode() == TargetOpcode::G_BUILD_VECTOR);
+
+  unsigned UnmergeUseCount = 0;
+  unsigned UndefInstrCount = 0;
+
+  unsigned UnmergeEltCount = 0;
+  unsigned UnmergeEltSize = 0;
+
+  unsigned BuildOperandCount = MI.getNumOperands();
+  bool EncounteredUndef = false;
+
+  Register UnmergeSrcTemp;
+  MachineInstr *UnmergeInstr;
+
+  std::set<int> KnownRegs;
+
+  for (auto &Use : MI.all_uses()) {
+    auto *Def = getDefIgnoringCopies(Use.getReg(), MRI);
+
+    if (!Def) {
----------------
arsenm wrote:

This should not be possible 

https://github.com/llvm/llvm-project/pull/165539