Add a new reduction pattern match

Adding a new reduction pattern match for vectorizing code similar
to TSVC s3111:

for (int i = 0; i < N; i++)
  if (a[i] > b)
    sum += a[i];

This patch adds support for fadd, fsub and fmull, as well as multiple
branches and different (but compatible) instructions (ex. add+sub) in
different branches.

The difference from the previous patch(https://reviews.llvm.org/D49168)
is as follows:
 - Added check of fast-math property of fp-instruction to the
   previous patch
 - Fix/add some pattern for if-reduction.ll


Differential Revision: https://reviews.llvm.org/D54464

Patch by Takahiro Miyoshi <takahiro.miyoshi@linaro.org>
     and Masakazu Ueno <masakazu.ueno@linaro.org>

llvm-svn: 347989