[DeLICM] Partial writes for PHIs.

Enable the use for partial writes for PHI write accesses with a switch.
This simply skips the test for whether a PHI write would be partial.

The analog test for partial value writes also protects for partial reads
which we do not support (yet). It is possible to test for partial reads
separately such that we could skip the partial write check as well. In
case this shows up to be useful, I can implement it as well.

Differential Revision: https://reviews.llvm.org/D33487

llvm-svn: 303762