aarch64: Detect scalar extending loads

aarch64: Detect scalar extending loads

If the scalar code does an integer load followed by an integer
extension, we've tended to cost that as two separate operations,
even though the extension is probably going to be free in practice.
This patch treats the extension as having zero cost, like we already
do for extending SVE loads.

Like with previous patches, this one only becomes active if
a CPU selects use_new_vector_costs.  It should therefore have
a very low impact on other CPUs.

gcc/
	* config/aarch64/aarch64.c (aarch64_detect_scalar_stmt_subtype):
	New function.
	(aarch64_add_stmt_cost): Call it.