[Polly] Remove -polly-vectorizer=polly.

Polly's internal vectorizer is not well maintained and is known to not work in some cases such as region ScopStmts. Unlike LLVM's LoopVectorize pass it also does not have a target-dependent cost heuristics, and we recommend using LoopVectorize instead of -polly-vectorizer=polly.

In the future we hope that Polly can collaborate better with LoopVectorize, like Polly marking a loop is safe to vectorize with a specific simd width, instead of replicating its functionality.

Reviewed By: grosser

Differential Revision: https://reviews.llvm.org/D142640