[ARM] Add missing selection patterns for vnmla

For the following function:

  double fn1(double d0, double d1, double d2) {
    double a = -d0 - d1 * d2;
    return a;
  }

on ARM, LLVM generates code along the lines of

  vneg.f64  d0, d0
  vmls.f64  d0, d1, d2

i.e., a negate and a multiply-subtract.

The attached patch adds instruction selection patterns to allow it to generate the single instruction

  vnmla.f64  d0, d1, d2

(multiply-add with negation) instead, like GCC does.

Committed on behalf of @gergo- (Gergö Barany)

Differential Revision: https://reviews.llvm.org/D35911

llvm-svn: 313972