target/riscv: rvv-1.0: Add ELEN checks for widening and narrowing instructions

SEW has the limitation which cannot exceed ELEN.

Widening instructions have a destination group with EEW = 2*SEW
and narrowing instructions have a source operand with EEW = 2*SEW.
Both of the instructions have the limitation of: 2*SEW <= ELEN.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-78-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>