[MemCpyOptimizer] Simplify API of processStore and processMem* functions

Previously these functions either returned a "changed" flag or a "repeat
instruction" flag, and could also modify an iterator to control which
instruction would be processed next.

Simplify this by always returning a "changed" flag, and handling all of
the "repeat instruction" functionality by modifying the iterator.

No functional change intended except in this case:
// If the source and destination of the memcpy are the same, then zap it.
... where the previous code failed to process the instruction after the
zapped memcpy.

Differential Revision: https://reviews.llvm.org/D81540