Add a new option -run-slp-after-loop-vectorization.

This swaps the order of the loop vectorizer and the SLP/BB vectorizers. It is disabled by default so we can do performance testing - ideally we want to change to having the loop vectorizer running first, and the SLP vectorizer using its leftovers instead of the other way around.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214963 91177308-0d34-0410-b5e6-96231b3b80d8
[AArch64] Add a testcase for r214957.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214965 91177308-0d34-0410-b5e6-96231b3b80d8