[DAGCombiner][X86] Improve narrowExtractedVectorLoad to handle cases where the element size isn't byte sized by the subvector is.
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
Differential Revision: https://reviews.llvm.org/D75434