[AArch64] Allow folding FMUL/FADD into FMA for FP16 types

isFMAFasterThanFMulAndFAdd should return true for FP16 types when
HasFullFP16 is present, since we have the instructions to handle it for
both SVE and NEON. (SVE patterns and tests will follow).

Differential Revision: https://reviews.llvm.org/D96599