Improve clamp recognition in ValueTracking.

Summary:
ValueTracking was recognizing not all variations of clamp. Swapping of
true value and false value of select was added to fix this problem. This
change breaks the canonical form of cmp inside the matchMinMax function,
that is why additional checks for compare predicates is needed. Added
corresponding test cases.

Reviewers: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38531

Patch by: Artur Gainullin <artur.gainullin@intel.com>

llvm-svn: 315992