aarch64: Fix FMV array iteration bounds

aarch64: Fix FMV array iteration bounds

There was an assumption in some places that the aarch64_fmv_feature_data
array contained FEAT_MAX elements.  While this assumption held up till
now, it is safer and more flexible to use the array size directly.

Also fix the lower bound in compare_feature_masks to use ">=0" instead
of ">0", and add a test using the features at index 0 and 1. However,
the test already passed, because the earlier popcount check makes it
impossible to reach the loop if the masks differ in exactly one
location.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (compare_feature_masks):
	Use ARRAY_SIZE and >=0 for iteration bounds.
	(aarch64_mangle_decl_assembler_name): Use ARRAY_SIZE.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/mv-1.C: New test.