RISC-V: Fix Werror=sign-compare in riscv_validate_vector_type

This patch would like to fix the Werror=sign-compare similar to below:

gcc/config/riscv/riscv.cc: In function ‘void
riscv_validate_vector_type(const_tree, const char*)’:
gcc/config/riscv/riscv.cc:5614:23: error: comparison of integer
expressions of different signedness: ‘int’ and ‘unsigned int’
[-Werror=sign-compare]
 5614 |   if (TARGET_MIN_VLEN < required_min_vlen)

The TARGET_MIN_VLEN is *int* by default but the required_min_vlen
returned from riscv_vector_required_min_vlen is **unsigned**.  Thus,
adjust the related function and reference variable(s) to int type
to avoid such kind of Werror.

The below test suite is passed for this patch.
* The rv64gcv fully regression tests.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_vector_float_type_p): Take int
	as the return value instead of unsigned.
	(riscv_vector_element_bitsize): Ditto.
	(riscv_vector_required_min_vlen): Ditto.
	(riscv_validate_vector_type): Take int type for local variable(s).

Signed-off-by: Pan Li <pan2.li@intel.com>