[SROA] Clean up some uses of MaybeAlign in SROA.

Use Align instead of using MaybeAlign; all the operations in question
have known alignment.

For getSliceAlign() in particular, in the cases where we used to return
None, it would be converted back to an Align by IRBuilder, so there's no
functional change there.

Split off from D77454.

Differential Revision: https://reviews.llvm.org/D79205