[JumpThreading] Add a pattern to TryToUnfoldSelectInCurrBB()

[JumpThreading] Add a pattern to TryToUnfoldSelectInCurrBB()

Add the following pattern to TryToUnfoldSelectInCurrBB()

bb:
   %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ...
   %c = cmp %p, 0
   %s = select %c, trueval, falseval

The Select in the above pattern will be unfolded and then jump-threaded. The
current implementation does not allow CMP in the middle of PHI and Select.

Differential Revision: https://reviews.llvm.org/D34762

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308050 91177308-0d34-0410-b5e6-96231b3b80d8