[RISCV] Make RISCVISAInfo::updateMaxELen extension checking more robust. Add...

[RISCV] Make RISCVISAInfo::updateMaxELen extension checking more robust. Add inference from V extension. (#90650)

We weren't fully checking that we parsed Zve*x/f/d correctly. This could
break if new extension is added that starts with Zve.

We were assuming the Zve64d is present whenever V is so we only
inferred from Zve*. It's more correct to infer ELEN from V itself too.