[DSE] Stores not eliminated if they are overwritten in a loop.

[fhahn]

Bugzilla Link	46460
Version	trunk
OS	All
CC	@vdsered,@whitneywhtsang

Extended Description

In the example below, LLVM currently does not eliminate the stores in the second loop, even though the second loop exactly overwrites the stores of the first loop. Similarly the first loop could be replaced by a memcpy and we still not remove it:

https://godbolt.org/z/kGBEXS

void init(int *P, unsigned N) {
   for (unsigned i = 0; i < N; i++)
        P[i] = 1;

    for (unsigned i = 0; i < N; i++)
        P[i] = 1;
}