[llvm] [RISCV] Handle fixed length vectors with exact VLEN in loweringEXTRACT_SUBVECTOR (PR #79949)

[Philip Reames via llvm-commits]

================
@@ -9653,19 +9656,46 @@ SDValue RISCVTargetLowering::lowerEXTRACT_SUBVECTOR(SDValue Op,
     return DAG.getBitcast(Op.getValueType(), Slidedown);
   }
 
+  if (VecVT.isFixedLengthVector()) {
+    VecVT = getContainerForFixedLengthVector(VecVT);
+    Vec = convertToScalableVector(VecVT, Vec, DAG, Subtarget);
+  }
+
+  MVT ContainerSubVecVT = SubVecVT;
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preames wrote:

Minor, but please move this defintion above the if guard just above, and add "ContainerSubVecVT = getContainerForFixedLengthVector(SubVecVT);" in the if clause.  I found this confusing on first read through.

https://github.com/llvm/llvm-project/pull/79949