aarch64: Fix gather x32/x64 selection

The SVE gather and scatter costs are classified based on whether
they do 4 loads per 128 bits (x32) or 2 loads per 128 bits (x64).
The number after the "x" refers to the number of bits in each
"container".

However, the test for which to use was based on the element size
rather than the container size.  This meant that we'd use the
overly conservative x32 costs for VNx2SI gathers.  VNx2SI gathers
are really .D gathers in which the upper half of each extension
result is ignored.

This patch is necessary to switch -mtune=generic over to the
"new" vector costs.

gcc/
	* config/aarch64/aarch64.cc (aarch64_detect_vector_stmt_subtype)
	(aarch64_vector_costs::add_stmt_cost): Use the x64 cost rather
	than x32 cost for all VNx2 modes.