aarch64: Account for different Advanced SIMD fusing options

The scalar FNMADD/FNMSUB and SVE FNMLA/FNMLS instructions mean
that either side of a subtraction can start an accumulator chain.
However, Advanced SIMD doesn't have an equivalent instruction.
This means that, for Advanced SIMD, a subtraction can only be
fused if the second operand is a multiplication.

Also, if both sides of a subtraction are multiplications,
and if the second operand is used multiple times, such as:

     c * d - a * b
     e * f - a * b

then the first rather than second multiplication operand will tend
to be fused.  On Advanced SIMD, this leads to:

     tmp1 = a * b
     tmp2 = -tmp1
      ... = tmp2 + c * d   // FMLA
      ... = tmp2 + e * f   // FMLA

where one of the FMLAs also requires a MOV.

This patch tries to account for this in the vector cost model.
It improves roms performance by 2-3% on Neoverse V1.  It's also
needed to avoid a regression in fotonik for Neoverse N2 and
Neoverse V2 with the patch for PR110625.

gcc/
	* config/aarch64/aarch64.cc: Include ssa.h.
	(aarch64_multiply_add_p): Require the second operand of an
	Advanced SIMD subtraction to be a multiplication.  Assume that
	such an operation won't be fused if the second operand is used
	multiple times and if the first operand is also a multiplication.

gcc/testsuite/
	* gcc.target/aarch64/neoverse_v1_2.c: New test.
	* gcc.target/aarch64/neoverse_v1_3.c: Likewise.

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