Riscv patterns to optimize away some redundant zero/sign extends.

Riscv patterns to optimize away some redundant zero/sign extends.

	gcc/
	* config/riscv/riscv.c (SINGLE_SHIFT_COST): New.
	(riscv_rtx_costs): Case ZERO_EXTRACT, match new pattern, and return
	SINGLE_SHIFT_COST.  Case LT and ZERO_EXTEND, likewise.  Case ASHIFT,
	use SINGLE_SHIFT_COST.
	* config/riscv/riscv.md (lshrsi3_zero_extend_1): New.
	(lshrsi3_zero_extend_2, lshrsi3_zero_extend_3): New.

	gcc/testsuite/
	* gcc.target/riscv/riscv.exp: New.
	* gcc.target/riscv/zero-extend-1.c: New.
	* gcc.target/riscv/zero-extend-2.c: New.
	* gcc.target/riscv/zero-extend-3.c: New.
	* gcc.target/riscv/zero-extend-4.c: New.

Co-Authored-By: Andrew Waterman <andrew@sifive.com>
From-SVN: r255257