[DAGCombiner][X86] Improve narrowExtractedVectorLoad to handle cases where the...

[DAGCombiner][X86] Improve narrowExtractedVectorLoad to handle cases where the element size isn't byte sized by the subvector is.

Summary:
Follow up from D75377. If the subvector is byte sized and the
index is aligned to the subvector size, we can shrink the load.

Reviewers: spatel, RKSimon

Reviewed By: RKSimon

Subscribers: dbabokin, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75434