[DSE] Only eliminate candidates that always store the same loc.

AliasAnalysis/MemoryLocation does not account for loops. Two
MemoryLocation can be must-overwrite, even if the first one writes
multiple locations in a loop.

This patch prevents removing such stores, by only considering candidates
that are known to be loop invariant, or executed in the same BB.

Currently the invariant check is quite conservative and only considers
Alloca and Alloca-like instructions and arguments as invariant base pointers.
It also considers GEPs with all constant indices and invariant bases as
invariant.

This can be improved in the future, but the current implementation has
only minor impact on the total number of stores eliminated (25903 vs
26047 for the baseline). There are some 2-10% swings for some individual
benchmarks. In roughly half of the cases, the number of stores removed
increases actually, because we skip candidates that are unlikely to be
valid candidates early.