[PATCH] Fix interference caused by fmul 2, x -> fadd x, x combine

[Matt Arsenault]

This solves 2 variants of this problem. First, change the order things are tried so that fmul (fmul x, c1) c2 -> fmul x, (fmul c1, c2) before fadd x, x.

Also add a variant of the fmul constant combine that understands fadd x, x as a multiply by 2. This is necessary because a multiply by 2 that exists originally will be transformed into the fadd by one of the early runs of DAG combiner, and not folded with new fmuls inserted during lowering.

http://reviews.llvm.org/D4678

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/R600/llvm.sin.ll
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