[GlobalOpt] recompute alignments for loads and stores of updated globals

GlobalOpt can slice structs/arrays and change GEPs in the process,
but it was not updating alignments for load/store users. This
eventually causes the crashing seen in:
https://llvm.org/PR49661
https://llvm.org/PR50253

On x86, this required SLP+codegen to create an aligned vector
store on an invalid address. The bugs would be easier to
demonstrate on a target with stricter alignment requirements.

I'm not sure if this is a complete solution. The alignment
updating code is adapted from InstCombine, so I assume that
part is tested and good.

Differential Revision: https://reviews.llvm.org/D102552