RISC-V: Add zero_extract support for rv64gc

RISC-V: Add zero_extract support for rv64gc

The combiner attempts to optimize a zero-extension of a logical right shift
using zero_extract. We already utilize this optimization for those cases
that result in a single instructions.  Let's add a insn_and_split
pattern that also matches the generic case, where we can emit an
optimized sequence of a slli/srli.

Tested with SPEC CPU 2017 (rv64gc).

	PR target/111501

gcc/ChangeLog:

	* config/riscv/riscv.md (*lshr<GPR:mode>3_zero_extend_4): New
	pattern for zero-extraction.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/extend-shift-helpers.h: New test.
	* gcc.target/riscv/pr111501.c: New test.
	* gcc.target/riscv/zero-extend-rshift-32.c: New test.
	* gcc.target/riscv/zero-extend-rshift-64.c: New test.
	* gcc.target/riscv/zero-extend-rshift.c: New test.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>