[ScalarizeMaskedMemIntrin][SelectionDAGBuilder] Use the element type to...

[ScalarizeMaskedMemIntrin][SelectionDAGBuilder] Use the element type to calculate alignment for gather/scatter when alignment operand is 0.

Previously we used the vector type, but we're loading/storing
invididual elements so I think only element alignment should matter.

Noticed while looking at the code for something else so I don't
have a test case.

Differential Revision: https://reviews.llvm.org/D105220