Remove TargetBaseAlign. Keep alignment for stack adjustments.

Summary:
TargetBaseAlign is no longer required since LSV checks if target allows misaligned accesses.
A constant defining a base alignment is still needed for stack accesses where alignment can be adjusted.

Reviewers: llvm-commits, jlebar

Subscribers: mzolotukhin, arsenm

Differential Revision: https://reviews.llvm.org/D22936

llvm-svn: 277038