target/riscv: introduce more imm value modes in translator functions

target/riscv: introduce more imm value modes in translator functions

Immediate value in translator function is extended not only
zero-extended and sign-extended but with more modes to be applicable
with multiple formats of vector instructions.

* IMM_ZX:         Zero-extended
* IMM_SX:         Sign-extended
* IMM_TRUNC_SEW:  Truncate to log(SEW) bit
* IMM_TRUNC_2SEW: Truncate to log(2*SEW) bit

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-17-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>