LoopVectorizer: Refactor the code that checks if it is safe to predicate blocks.

LoopVectorizer:  Refactor the code that checks if it is safe to predicate blocks.
In this code we keep track of pointers that we are allowed to read from, if they are accessed by non-predicated blocks.
We use this list to allow vectorization of conditional loads in predicated blocks because we know that these addresses don't segfault.

llvm-svn: 185214