Add straight-line strength reduction to LLVM

Add straight-line strength reduction to LLVM

Summary:
Straight-line strength reduction (SLSR) is implemented in GCC but not yet in
LLVM. It has proven to effectively simplify statements derived from an unrolled
loop, and can potentially benefit many other cases too. For example,

LLVM unrolls

  #pragma unroll
  foo (int i = 0; i < 3; ++i) {
    sum += foo((b + i) * s);
  }

into

  sum += foo(b * s);
  sum += foo((b + 1) * s);
  sum += foo((b + 2) * s);

However, no optimizations yet reduce the internal redundancy of the three
expressions:

  b * s
  (b + 1) * s
  (b + 2) * s

With SLSR, LLVM can optimize these three expressions into:

  t1 = b * s
  t2 = t1 + s
  t3 = t2 + s

This commit is only an initial step towards implementing a series of such
optimizations. I will implement more (see TODO in the file commentary) in the
near future. This optimization is enabled for the NVPTX backend for now.
However, I am more than happy to push it to the standard optimization pipeline
after more thorough performance tests.

Test Plan: test/StraightLineStrengthReduce/slsr.ll

Reviewers: eliben, HaoLiu, meheff, hfinkel, jholewinski, atrick

Reviewed By: jholewinski, atrick

Subscribers: karthikthecool, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D7310

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228016 91177308-0d34-0410-b5e6-96231b3b80d8