target/riscv: add gen_shifti() and gen_shiftiw() helper functions

target/riscv: add gen_shifti() and gen_shiftiw() helper functions

Add gen_shifti() and gen_shiftiw() helper functions to reuse the same
interfaces for immediate shift instructions.

Signed-off-by: Frank Chang <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-id: 20210505160620[email protected]
Signed-off-by: Alistair Francis <[email protected]>