[DSE] Check for whole object overwrite even if dead store size not known

If the killing store overwrites the whole object, we know that the
preceding store is dead, regardless of the accessed offset or size.
This case was previously only handled if the size of the dead store
was also known.

This allows us to perform conventional DSE for calls that write to
an argument (but without known size).

Differential Revision: https://reviews.llvm.org/D116267