AArch64: update costing for combining vector conditionals

boolean comparisons have different cost depending on the mode. e.g.
for SVE, a && b doesn't require an additional instruction when a or b
is predicated by combining the predicate of the one operation into the
second one.  At the moment though we only fuse compares so this update
requires one of the operands to be a comparison.

Scalars also don't require this because the non-ifcvt variant is a series of
branches where following the branch sequences themselves are natural ANDs.

Advanced SIMD however does require an actual AND to combine the boolean values.

As such this patch discounts Scalar and SVE boolean operation latency and
throughput.

With this patch comparison heavy code prefers SVE as it should, especially in
cases with SVE VL == Advanced SIMD VL where previously the SVE prologue costs
would tip it towards Advanced SIMD.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_bool_compound_p): New.
	(aarch64_adjust_stmt_cost, aarch64_vector_costs::count_ops): Use it.

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