[RISCV] Make the hasStdExtM() check in RISCVInstrInfo::getVLENFactoredAmount...

[RISCV] Make the hasStdExtM() check in RISCVInstrInfo::getVLENFactoredAmount emit a diagnostic rather than an assert.

As far as I know we're not enforcing the StdExtM must be enabled
to use the V extension. If we use an assert here and hit this
code in a release build we'll silently emit an invalid instruction.

By using a diagnostic we report the error to the user in release
builds. I think there may still be a later fatal error from
the code emitter though.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D97970