[LV] 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.

I have forwarded to trunk, added fsub and fmul functionality and
additional tests, but the credit goes to Takahiro, who did most of the
actual work.

Differential Revision: https://reviews.llvm.org/D49168

Patch by Takahiro Miyoshi <takahiro.miyoshi@linaro.org>.

llvm-svn: 344172