[AArch64] Combine fadd into fcmla

This adds a target combine for `fadd(a, vcmla(b, c, d))` -> `vcmla(fadd(a, b), b, c)`,
pushing the fadd into the operands of the fcmla, which can help simplify away some
additions.

Differential Revision: https://reviews.llvm.org/D146407