target/riscv: Fix LMUL check to use VLEN

target/riscv: Fix LMUL check to use VLEN

The previous check was failing with:

VLEN=128 ELEN = 64 SEW = 16 and LMUL = 1/8 which is a
valid combination.

Fix the check to allow valid combinations when VLEN is a multiple of
ELEN.

From the specification:

"In general, the requirement is to support LMUL ≥ SEWMIN/ELEN, where
SEWMIN is the narrowest supported SEW value and ELEN is the widest
supported SEW value. In the standard extensions, SEWMIN=8. For standard
vector extensions with ELEN=32, fractional LMULs of 1/2 and 1/4 must be
supported. For standard vector extensions with ELEN=64, fractional LMULs
of 1/2, 1/4, and 1/8 must be supported." Elsewhere in the specification
it makes clear that VLEN>=ELEN.

From inspection this new check allows:

VLEN=ELEN=64 1/2, 1/4, 1/8 for SEW >=8
VLEN=ELEN=32 1/2, 1/4 for SEW >=8

Fixes: d9b7609a1fb2 ("target/riscv: rvv-1.0: configure instructions")
Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Message-Id: <20230718131316.12283-2-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>