With a sized dead_on_return, we need to not eliminate stores if there
are to a pointer with a variable offset from the underlying object
marked dead_on_return. This manifested as an assertion failure as
BaseValue/V ended up not being equal. It's possible we could do a range
analysis to try and prove the variable offset stays within bounds, but
this case seems to come up relatively rarely (only reproducible with a
UBSan build of LLVM) and is probably not worth the compile time.
Fixes#180361.
ec059d81aa