[Mem2Reg] Don't canonicalize erased vals.
To eliminate copies which become newly spurious, Mem2Reg canonicalizes
the lifetimes of values that are stored and of newly introduced phis
after rewriting.
It's possible, however, for the values that are stored to be deleted
during canonicalization if a value and its copy are both stored to the
address. Such values must not be canonicalized. So check whether
values have been erased before canonicalizing them.
rdar://113762355
