[DSE,MemorySSA] Limit elimination at end of function to single UO.

Limit elimination of stores at the end of a function to MemoryDefs with
a single underlying object, to save compile time.

In practice, the case with multiple underlying objects seems not very
important in practice. For -O3 -flto on MultiSource/SPEC2000/SPEC2006
this results in a total of 2 more stores being eliminated.

We can always re-visit that in the future.