Improve lookThroughCast function.

Summary:
When we have the following case:

  %cond = cmp iN %x, CmpConst
  %tr = trunc iN %x to iK
  %narrowsel = select i1 %cond, iK %t, iK C

We could possibly match only min/max pattern after looking through cast.
So it is more profitable if widened C constant will be equal CmpConst.
That is why just set widened C constant equal to CmpConst, because there
is a further check in this function that trunc CmpConst == C.

Also description for lookTroughCast function was added.

Reviewers: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38536

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

llvm-svn: 316070