aarch64: Tweak MLA vector costs

The issue-based vector costs currently assume that a multiply-add
sequence can be implemented using a single instruction.  This is
generally true for scalars (which have a 4-operand instruction)
and SVE (which allows the output to be tied to any input).
However, for Advanced SIMD, multiplying two values and adding
an invariant will end up being a move and an MLA.

The only target to use the issue-based vector costs is Neoverse V1,
which would generally prefer SVE in this case anyway.  I therefore
don't have a self-contained testcase.  However, the distinction
becomes more important with a later patch.

gcc/
	* config/aarch64/aarch64.c (aarch64_multiply_add_p): Add a vec_flags
	parameter.  Detect cases in which an Advanced SIMD MLA would almost
	certainly require a MOV.
	(aarch64_count_ops): Update accordingly.

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