[GlobalOpt] Fix a miscompile when evaluating struct initializers.

The bug was that evaluateBitcastFromPtr attempts a narrowing to a struct's 0th
element of a store that covers other elements. While this is okay on the load
side, applying it to stores causes us to miss the writes to the additionally
covered elements.

rdar://79503568

Differential revision: https://reviews.llvm.org/D105838