RISCV: Add rotate immediate regression test

This adds new regression tests to ensure half-register rotations are
correctly optimized into rori instructions.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zbb-rol-ror-08.c: New test.
	* gcc.target/riscv/zbb-rol-ror-09.c: New test.

Co-authored-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>