[LoopVectorize] Don't consider conditional-load dereferenceability for marked parallel loops

I really thought we were doing this already, but we were not. Given this input:

void Test(int *res, int *c, int *d, int *p) {
  for (int i = 0; i < 16; i++)
    res[i] = (p[i] == 0) ? res[i] : res[i] + d[i];
}

we did not vectorize the loop. Even with "assume_safety" the check that we
don't if-convert conditionally-executed loads (to protect against
data-dependent deferenceability) was not elided.

One subtlety: As implemented, it will still prefer to use a masked-load
instrinsic (given target support) over the speculated load. The choice here
seems architecture specific; the best option depends on how expensive the
masked load is compared to a regular load. Ideally, using the masked load still
reduces unnecessary memory traffic, and so should be preferred. If we'd rather
do it the other way, flipping the order of the checks is easy.

The LangRef is updated to make explicit that llvm.mem.parallel_loop_access also
implies that if conversion is okay.

Differential Revision: http://reviews.llvm.org/D19512

llvm-svn: 267514