[CodeGen] Support partial write accesses.

Allow the BlockGenerator to generate memory writes that are not defined
over the complete statement domain, but only over a subset of it. It
generates a condition that evaluates to 1 if executing the subdomain,
and only then execute the access.

Only write accesses are supported. Read accesses would require a PHINode
which has a value if the access is not executed.

Partial write makes DeLICM able to apply mappings that are not defined
over the entire domain (for instance, a branch that leaves a loop with
a PHINode in its header; a MemoryKind::PHI write when leaving is never
read by its PHI read).

Differential Revision: https://reviews.llvm.org/D33255

llvm-svn: 303517