Extend late diagnostics to include late test for runtime pointer checks.

Extend late diagnostics to include late test for runtime pointer checks.

This patch moves checking the threshold of runtime pointer checks to the vectorization requirements (late diagnostics) and emits a diagnostic that infroms the user the loop would be vectorized if not for exceeding the pointer-check threshold. Clang will also append the options that can be used to allow vectorization.

llvm-svn: 244523