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

[Amara Emerson via llvm-commits]

================
@@ -3463,6 +3464,89 @@ static bool isConstValidTrue(const TargetLowering &TLI, unsigned ScalarSizeBits,
          isConstTrueVal(TLI, Cst, IsVector, IsFP);
 }
 
+// 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 CombinerHelper::matchCombineBuildUnmerge(MachineInstr &MI,
+                                              MachineRegisterInfo &MRI,
+                                              Register &UnmergeSrc) const {
+  assert(MI.getOpcode() == TargetOpcode::G_BUILD_VECTOR);
----------------
aemerson wrote:

If you do:
```
auto &BV = cast<GBuildVector>(MI);
```
You can skip this assert and get some convenience helpers.

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