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. The
first patch was reverted because it caused miscompile in NVPTX target. 
Added corresponding test cases.

Reviewers: spatel, majnemer, efriedma, reames

Subscribers: llvm-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D39240

llvm-svn: 316795