Add tests to document current InstCombine behavior for clamp pattern.

Summary:
This commit adds the tests for clamp pattern as a prerequisite of
D33186 to make the impact of that fix more clear and also to document
current behavior.

Reviewers: spatel, jmolloy

Reviewed By: spatel

Subscribers: n.bozhenov, llvm-commits

Patch by Andrei Elovikov <andrei.elovikov@intel.com>

Differential Revision: https://reviews.llvm.org/D34350

llvm-svn: 306524