AlignmentFromAssumptions and SLPVectorizer preserves AA and GlobalsAA

GlobalsAA's assumptions that passes do not escape globals not previously
escaped is not violated by AlignmentFromAssumptions and SLPVectorizer. Marking
them as such allows GlobalsAA to be preserved until GVN in the LTO pipeline.

http://lists.llvm.org/pipermail/llvm-dev/2015-December/092972.html

Patch by Vaivaswatha Nagaraj!

llvm-svn: 255348