[InstCombine] Recognize and simplify three way comparison idioms

[InstCombine] Recognize and simplify three way comparison idioms

Summary:
Many languages have a three way comparison idiom where comparing two values
produces not a boolean, but a tri-state value. Typical values (e.g. as used in
the lcmp/fcmp bytecodes from Java) are -1 for less than, 0 for equality, and +1
for greater than.

We actually do a great job already of converting three way comparisons into
binary comparisons when the result produced has one a single use. Unfortunately,
such values can have more than one use, and in that case, our existing
optimizations break down.

The patch adds a peephole which converts a three-way compare + test idiom into a
binary comparison on the original inputs. It focused on replacing the test on
the result of the three way compare and does nothing about removing the three
way compare itself. That's left to other optimizations (which do actually kick
in commonly.)
We currently recognize one idiom on signed integer compare. In the future, we
plan to recognize and simplify other comparison idioms on
other signed/unsigned datatypes such as floats, vectors etc.

This is a resurrection of Philip Reames' original patch:
https://reviews.llvm.org/D19452

Reviewers: majnemer, apilipenko, reames, sanjoy, mkazantsev

Reviewed by: mkazantsev

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34278

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306100 91177308-0d34-0410-b5e6-96231b3b80d8