Select lower fsub,fabs pattern to fabd on AArch64

This patch lowers patterns such as-
  fsub   v0.4s, v0.4s, v1.4s
  fabs   v0.4s, v0.4s
to
  fabd  v0.4s, v0.4s, v1.4s
on AArch64.

Review: http://reviews.llvm.org/D6791
llvm-svn: 225169