AArch64: Update costing for vector conversions [PR110625]

In gimple the operation

short _8;
double _9;
_9 = (double) _8;

denotes two operations on AArch64.  First we have to widen from short to
long and then convert this integer to a double.

Currently however we only count the widen/truncate operations:

(double) _5 6 times vec_promote_demote costs 12 in body
(double) _5 12 times vec_promote_demote costs 24 in body

but not the actual conversion operation, which needs an additional 12
instructions in the attached testcase.   Without this the attached testcase ends
up incorrectly thinking that it's beneficial to vectorize the loop at a very
high VF = 8 (4x unrolled).

Because we can't change the mid-end to account for this the costing code in the
backend now keeps track of whether the previous operation was a
promotion/demotion and ajdusts the expected number of instructions to:

1. If it's the first FLOAT_EXPR and the precision of the lhs and rhs are
   different, double it, since we need to convert and promote.
2. If it's the previous operation was a demonition/promotion then reduce the
   cost of the current operation by the amount we added extra in the last.

with the patch we get:

(double) _5 6 times vec_promote_demote costs 24 in body
(double) _5 12 times vec_promote_demote costs 36 in body

which correctly accounts for 30 operations.

This fixes the 16% regression in imagick in SPECCPU 2017 reported on Neoverse N2
and using the new generic Armv9-a cost model.

gcc/ChangeLog:

	PR target/110625
	* config/aarch64/aarch64.cc (aarch64_vector_costs::add_stmt_cost):
	Adjust throughput and latency calculations for vector conversions.
	(class aarch64_vector_costs): Add m_num_last_promote_demote.

gcc/testsuite/ChangeLog:

	PR target/110625
	* gcc.target/aarch64/pr110625_4.c: New test.
	* gcc.target/aarch64/sve/unpack_fcvt_signed_1.c: Add
	--param aarch64-sve-compare-costs=0.
	* gcc.target/aarch64/sve/unpack_fcvt_unsigned_1.c: Likewise