[ScalarizeMaskedMemIntrin] Scalarize constant mask expandload as shuffle(build_vector,pass_through)

As noticed on D66004, scalarization of an expandload with a constant mask as a chain of irregular loads+inserts makes it tricky to optimize before lowering, resulting in difficulties in merging loads etc.

This patch instead scalarizes the expansion to a build_vector(load0, load1, undef, load2,....) style pattern and then performs a blend shuffle with the pass through vector. This allows us to more easily make use of all the build_vector combines, merging of consecutive loads etc.

Differential Revision: https://reviews.llvm.org/D85416

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