Revert "Always treat scalar writes as MUST_WRITEs"

This reverts commit r255471.

Johannes raised in the post-commit review of r255471 the concern that PHI
writes in non-affine regions with two exiting blocks are not really MUST_WRITE,
but we just know that at least one out of the set of all possible PHI writes
will be executed. Modeling all PHI nodes as MUST_WRITEs is probably save, but
adding the needed documentation for such a special case is probably not worth
the effort. Michael will be proposing a new patch that ensures only a single
PHI_WRITE is created for non-affine regions, which - besides other benefits -
should also allow us to use a single well-defined MUST_WRITE for such PHI
writes.

(This is not a full revert, but the condition and documentation have been
slightly extended)

llvm-svn: 255503