[InstCombine] Add a new formula for SMIN.

Summary:
After this change `MatchSelectPattern` recognizes the following form
of SMIN:

  Y >s C ? ~Y : ~C == ~Y <s ~C ? ~Y : ~C = SMIN(~Y, ~C)

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9352

llvm-svn: 236202