vect: Pass mode to gather/scatter tests

vect: Pass mode to gather/scatter tests

vect_check_gather_scatter had a binary “does this target support
internal gather/scatter functions” test.  This dates from the time when
we only handled gathers and scatters via direct target support, with
x86_64 using built-in functions and aarch64 using IFNs.  But now that we
can emulate gathers, we need to check whether the gather for a particular
mode is going to be emulated or not.

Without this, enabling SVE regresses emulated Advanced SIMD gather
sequences in cases where SVE isn't used.

Livermore kernel 15 can now be vectorised with Advanced SIMD when
SVE is enabled.

gcc/
	* genopinit.c (main): Turn supports_vec_gather_load and
	supports_vec_scatter_store into signed char arrays and remove
	supports_vec_gather_load_cached and supports_vec_scatter_store_cached.
	* optabs-query.c (supports_vec_convert_optab_p): Add a mode parameter.
	If the mode is not VOIDmode, test only for that mode.
	(supports_vec_gather_load_p): Likewise.
	(supports_vec_scatter_store_p): Likewise.
	* optabs-query.h (supports_vec_gather_load_p): Likewise.
	(supports_vec_scatter_store_p): Likewise.
	* tree-vect-data-refs.c (vect_check_gather_scatter): Pass the
	vector mode to supports_vec_gather_load_p and
	supports_vec_scatter_store_p.

gcc/testsuite/
	* gfortran.dg/vect/vect-8.f90: Bump number of vectorized loops
	to 25 for SVE.
	* gcc.target/aarch64/sve/gather_load_10.c: New test.