RISC-V: Add vectorized binops and insn_expander helpers.

RISC-V: Add vectorized binops and insn_expander helpers.

This patch adds basic binary integer operations support.  It is based
on Michael Collison's work and makes use of the existing helpers in
riscv-c.cc.  It introduces emit_nonvlmax_binop which, in turn, uses
emit_pred_binop.  Setting the destination as well as the mask and the
length are factored out into separate functions.

gcc/ChangeLog:

	* config/riscv/autovec.md (<optab><mode>3): Add integer binops.
	* config/riscv/riscv-protos.h (emit_nonvlmax_binop): Declare.
	* config/riscv/riscv-v.cc (emit_pred_op): New function.
	(set_expander_dest_and_mask): New function.
	(emit_pred_binop): New function.
	(emit_nonvlmax_binop): New function.

Co-authored-by: Michael Collison <collison@rivosinc.com>