[DeLICM] Use Known information when comparing Occupied and Written.

Do not conflict if a write writes the same value as already known.

This change only affects unit tests, but no functional changes are
expected on LLVM-IR, as no Known information is yet extracted and
consequently this functionality is only triggered through unit tests.

Differential Revision: https://reviews.llvm.org/D32026

llvm-svn: 301460