[AArch64] Change order of candidate FMLS patterns

r319980 added new patterns to the machine combiner for transforming (fsub (fmul

x y) z) into (fmla (fneg z) x y). That is, fsub's where the first source

operand is an fmul are transformed. We previously only matched the case where

the second source operand of an fsub was an fmul, transforming (fsub z (fmul x

y)) into (fmls z x y). Now, if we have an fsub where both source operands are

fmuls, both of the above patterns are applicable.

However, the order in which we add the patterns to the list of candidates

determines the transformation that takes place, since only the first pattern

that matches will be used. This patch changes the order these two patterns are

added to the list of candidates such that we prefer the case where the second

source operand is an fmul (the fmls case), rather than the other one (the

fmla/fneg case). When both source operands are fmuls, this ordering results in

fewer instructions.

Differential Revision: https://reviews.llvm.org/D41587