IfConversion: Fix branch predication bug.

This bug shows up with diamonds that share unpredicable, unanalyzable branches.
There's an included test case from Hexagon. What was happening was that we were
attempting to predicate the branch instruction despite the fact that it was
checked to be the same. Now for unanalyzable branches we skip over the branch
instructions when predicating the block.

Differential Revision: https://reviews.llvm.org/D23939

llvm-svn: 279985