[ARM] Combine fadd into fcmla

This is the MVE equivalent of https://reviews.llvm.org/D146407. It adds a
target combine for fadd(a, vcmla(b, c, d)) -> vcmla(fadd(a, b), c, d), pushing
the fadd into the operands of the fcmla, which can help simplify away some
additions.

Differential Revision: https://reviews.llvm.org/D147200