[ubsan] Don't check alignment if the alignment is 1

If a pointer is 1-byte aligned, there's no use in checking its
alignment. Somewhat surprisingly, ubsan can spend a significant amount
of time doing just that!

This loosely depends on D30283.

Testing: check-clang, check-ubsan, and a stage2 ubsan build.

Differential Revision: https://reviews.llvm.org/D30285

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300371 91177308-0d34-0410-b5e6-96231b3b80d8