[DeLICM] Properly handle PHI writes becoming empty partial writes.

It is possible that partial writes are empty (write is never executed).
In this case, when in PHINode's incoming edge is never taken such that
the incoming write becomes an empty partial write, if enabled. The
issue is that when converting the union_map to an map, it's space
cannot be derived from the union_map itself. Rather, we need to
determine its space independently.

This fixes test-suite's MultiSource/Benchmarks/ASC_Sequoia/CrystalMk.

llvm-svn: 310348