[DeLICM] Lookup input accesses.

Previous to this patch, we used VirtualUse to determine the input
access of an llvm::Value in a statement. The input access is the
READ MemoryAccess that makes a value available in that statement,
which can either be a READ of a MemoryKind::Value or the
MemoryKind::PHI for a PHINode in the statement. DeLICM uses the input
access to heuristically find a candidate to map without searching all
possible values.

This might modify the behaviour in that previously PHI accesses were
not considered input accesses before. This was unintentially lost when
"VirtualUse" was extracted from the "Known Knowledge" patch.

llvm-svn: 302838