RISC-V: Add vector_scalar_shift_operand

RISC-V: Add vector_scalar_shift_operand

The vector shift immediates happen to have the same constraints as some
of the CSR-related operands, but it's a different usage.  This adds a
name for them, so I don't get confused again next time.

gcc/ChangeLog:

	* config/riscv/autovec.md (shifts): Use
	vector_scalar_shift_operand.
	* config/riscv/predicates.md (vector_scalar_shift_operand): New
	predicate.