[WideableCond] Fix a nasty bug in detection of "explicit guards"

The code was failing to actually check for the presence of the call to widenable_condition.  The whole point of specifying the widenable_condition intrinsic was allowing widening transforms.  A normal branch is not widenable.  A normal branch leading to a deopt is not widenable (in general).

I added a test case via LoopPredication, but GuardWidening has an analogous bug.  Those are the only two passes actually using this utility just yet. Noticed while working on LoopPredication for non-widenable branches; POC in D60111.

llvm-svn: 357493