[SVE] Fix casts to <FixedVectorType> in truncateToMinimalBitwidths

[kmclaughlin]

Summary

Fixes more casts to <FixedVectorType> for the cases where the
instruction is a Insert/ExtractElementInst.

For fixed-width, this part of truncateToMinimalBitWidths is tested by
AArch64/type-shrinkage-insertelt.ll. I attempted to write a test case for this part
of truncateToMinimalBitWidths which uses scalable vectors, but was unable to add
one. The tests in type-shrinkage-insertelt.ll rely on scalarization to create extract
element instructions for instance, which is not possible for scalable vectors.

[david-arm]

LGTM! There are no tests attached here because there is currently no codepath where we can hit this for scalable vectors, but from a conceptual point of view there is no reason why it shouldn't work for scalable vectors.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

4052–4053

nit: Before merging/committing/submitting/<insert_correct_terminology here :)> can you fix up the formatting issues?

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

for (unsigned Part = 0; Part < UF; ++Part) {
         auto *O1 = B.CreateZExtOrTrunc(
             SI->getOperand(1), VectorType::get(ScalarTruncatedTy, Elements1));
 
         NewI = B.CreateShuffleVector(O0, O1, SI->getShuffleMask());
       } else if (isa<LoadInst>(I) || isa<PHINode>(I)) {
         // Don't do anything with the operands, just extend the result.
         continue;
       } else if (auto *IE = dyn_cast<InsertElementInst>(I)) {
-        auto Elements = cast<FixedVectorType>(IE->getOperand(0)->getType())
-                            ->getNumElements();
+        auto Elements =
+            cast<VectorType>(IE->getOperand(0)->getType())->getElementCount();
         auto *O0 = B.CreateZExtOrTrunc(
-            IE->getOperand(0),
-            FixedVectorType::get(ScalarTruncatedTy, Elements));
+            IE->getOperand(0), VectorType::get(ScalarTruncatedTy, Elements));
         auto *O1 = B.CreateZExtOrTrunc(IE->getOperand(1), ScalarTruncatedTy);
         NewI = B.CreateInsertElement(O0, O1, IE->getOperand(2));
       } else if (auto *EE = dyn_cast<ExtractElementInst>(I)) {
-        auto Elements = cast<FixedVectorType>(EE->getOperand(0)->getType())
-                            ->getNumElements();
+        auto Elements =
+            cast<VectorType>(EE->getOperand(0)->getType())->getElementCount();
         auto *O0 = B.CreateZExtOrTrunc(
-            EE->getOperand(0),
-            FixedVectorType::get(ScalarTruncatedTy, Elements));
+            EE->getOperand(0), VectorType::get(ScalarTruncatedTy, Elements));
         NewI = B.CreateExtractElement(O0, EE->getOperand(2));
       } else {
         // If we don't know what to do, be conservative and don't do anything.
         continue;
       }
 
       // Lastly, extend the result.
       NewI->takeName(cast<Instruction>(I));