Moving CGF::EmitAlignmentAssumption to IRBuilder

The functionality contained in CodeGenFunction::EmitAlignmentAssumption has
been moved to IRBuilder (so that it can also be used by LLVM-level code).
Remove this now-duplicate implementation in favor of the IRBuilder code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219877 91177308-0d34-0410-b5e6-96231b3b80d8