Combine the FADD(A, FMA(B, C, 0)) to FMA(B, C, A) and combine FADD(A, FMUL(B, C)...

Combine the FADD(A, FMA(B, C, 0)) to FMA(B, C, A) and combine FADD(A, FMUL(B, C)) to FMA(B, C, A).

This patch is to support transform in fast-math something like
_mm512_add_ph(x1, _mm512_fmadd_pch(a, b, _mm512_setzero_ph())) to
 _mm512_fmadd_pch(a, b, x1).

And support transform _mm512_add_ph(x1, _mm512_fmul_pch(a, b))
to _mm512_fmadd_pch(a, b, x1).

gcc/ChangeLog:

	* config/i386/sse.md (fma_<mode>_fadd_fmul): Add new
	define_insn_and_split.
	(fma_<mode>_fadd_fcmul):Likewise
	(fma_<complexopname>_<mode>_fma_zero):Likewise

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512fp16-complex-fma.c: New test.