RISC-V: Add patterns to convert AND mask to two shifts.

RISC-V: Add patterns to convert AND mask to two shifts.

	gcc/
	* config/riscv/predicates.md (p2m1_shift_operand): New.
	(high_mask_shift_operand): New.
	* config/riscv/riscv.md (lshrsi3_zero_extend_3+1): New combiner
	pattern using p2m1_shift_operand.
	(lshsi3_zero_extend_3+2): New combiner pattern using
	high_mask_shift_operand.

	gcc/testsuite/
	* gcc.target/riscv/shift-shift-1.c: New.
	* gcc.target/riscv/shift-shift-2.c: New.
	* gcc.target/riscv/shift-shift-3.c: New.

From-SVN: r262278