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.
Revert "r219834 - Teach ScalarEvolution to sharpen range information"

This change breaks the asan buildbots:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/13468