aarch64: Fix sve/acle/general/ldff1_8.c failures

gcc.target/aarch64/sve/acle/general/ldff1_8.c and
gcc.target/aarch64/sve/ptest_1.c were failing because the
aarch64 port was giving a zero (unknown) cost to instructions
that compute two results in parallel.  This was latent until
r15-1575-gea8061f46a30, which fixed rtl-ssa to treat zero costs
as unknown.

A long-standing todo here is to make insn_cost derive costs from md
information, rather than having to write a lot of matching code in
aarch64_rtx_costs.  But that's not something we can do for GCC 15.

This patch instead treats the cost of a PARALLEL as being the maximum
cost of its constituent sets.  I don't like this very much, since it
isn't really target-specific behaviour.  If it were stage 1, I'd be
trying to change pattern_cost instead.

gcc/
	* config/aarch64/aarch64.cc (aarch64_insn_cost): Give PARALLELs
	the same cost as the costliest SET.