HexagonOptAddrMode::changeStore - fix null dereference warning (PR43463)

As detailed on PR43463, this fixes a static analyzer null dereference warning by sinking Changed = true into the if() blocks where the MIB is actually created.

I did a quick check that suggested that one of those if() blocks is always guaranteed to be hit (so we could change it to if-else), but this seems like a safer approach

Differential Revision: https://reviews.llvm.org/D73883