[ASan] Speed up stack trace unwinding for stacks of size 2.

Summary:
We don't need to do any work in this case - just take
the current PC and caller PC.

Reviewers: eugenis, ygribov

Reviewed By: eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2936

llvm-svn: 202845