[instcombine] Fold away memset/memmove from otherwise unused alloca

The motivation for this is that while both memcpyopt and dse will catch this case, both are limited by MSSA's walk back threshold when finding clobbers.  As such, if you have a memcpy of an otherwise dead alloca placed towards the end of a long basic block with lots of other memory instructions, it would be missed.  This is a bit undesirable for such an "obviously" useless bit of code.

As noted in comments, we should probably generalize instcombine's escape analysis peephole (see visitAllocInst) to allow read xor write.  Doing that would subsume this code in a more general way, but is also a more involved change.  For the moment, I went with the easiest fix.