[SLP][REVEC] Fix InsertElement scalar type in getEntryCost
When SLPReVec is enabled, getValueType returns the vector result type
for InsertElement instructions rather than the scalar element type. This
caused getEntryCost to propagate an incorrect ScalarTy (e.g. <4 x float>
instead of float) into getScalarizationOverhead and getWidenedType,
triggering an assertion failure and producing wrong cost estimates.
Narrow ScalarTy to its element type when costing vectorized
InsertElement entries whose inserted operands are scalars.
Fixes #191175.
Reviewers:
Pull Request: https://github.com/llvm/llvm-project/pull/191628
