[SimplifyCFG] Improve store speculation check

isSafeToSpeculateStore() looks for a preceding store to the same
location to make sure that introducing a new store of the same
value is safe. It currently bails on intervening mayHaveSideEffect()
instructions. However, I believe just checking mayWriteToMemory()
is sufficient there -- we just need to make sure that we know which
value was stored, we don't care if we can unwind in the meantime.

While looking into this, I started having some doubts about the
correctness of the transform with regard to thread safety. While
we don't try to hoist non-simple stores, I believe we also need
to make sure that the preceding store is simple as well. Otherwise
we could introduce a spurious non-atomic write after an atomic write
-- under our memory model this would result in a subsequent undef
atomic read, even if the second write stores the same value as the
first.

Example: https://alive2.llvm.org/ce/z/q_3YAL

Differential Revision: https://reviews.llvm.org/D106742

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