[InstCombine] Allow InstCombine to merge adjacent guards

Summary:
If there are two adjacent guards with different conditions, we can
remove one of them and include its condition into the condition of
another one. This patch allows InstCombine to merge them by the
following pattern:

    guard(a); guard(b) -> guard(a & b).

Reviewers: reames, apilipenko, igor-laevsky, anna, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29378

llvm-svn: 293778