[llvm] GlobalISel: Use m_GImplicitDef instead of getVRegDef opcode checks (PR #216513)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-globalisel

Author: Matt Arsenault (arsenm)

<details>
<summary>Changes</summary>

Co-authored-by: Claude (Opus 4.8) <noreply@<!-- -->anthropic.com>

---
Full diff: https://github.com/llvm/llvm-project/pull/216513.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp (+1-2) 
- (modified) llvm/lib/CodeGen/GlobalISel/Utils.cpp (+1-1) 


``````````diff
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 914993a581085..6f85390fd4ede 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -9731,8 +9731,7 @@ LegalizerHelper::lowerVECTOR_COMPRESS(llvm::MachineInstr &MI) {
 
   auto OutPos = MIRBuilder.buildConstant(IdxTy, 0);
 
-  bool HasPassthru =
-      MRI.getVRegDef(Passthru)->getOpcode() != TargetOpcode::G_IMPLICIT_DEF;
+  bool HasPassthru = !mi_match(Passthru, MRI, m_GImplicitDef());
 
   if (HasPassthru)
     MIRBuilder.buildStore(Passthru, StackPtr, PtrInfo, VecAlign);
diff --git a/llvm/lib/CodeGen/GlobalISel/Utils.cpp b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
index c555b67d9198c..36e427188c0b5 100644
--- a/llvm/lib/CodeGen/GlobalISel/Utils.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
@@ -1331,7 +1331,7 @@ std::optional<ValueAndVReg> getAnyConstantSplat(Register VReg,
 
     // If AllowUndef, treat undef as value that will result in a constant splat.
     if (!ElementValAndReg) {
-      if (AllowUndef && isa<GImplicitDef>(MRI.getVRegDef(Element)))
+      if (AllowUndef && mi_match(Element, MRI, m_GImplicitDef()))
         continue;
       return std::nullopt;
     }

``````````

</details>


https://github.com/llvm/llvm-project/pull/216513