[llvm-branch-commits] [TableGen] Improve error message for bad VTByHwMode in RegisterByHwMode (PR #171254)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/171254

Previously we would assert when a ValueTypeByHwMode was missing a case
for the current mode, now we report an error instead. Interestingly this
error only ocurrs when the DAG patterns use RegClassByHwMode, but not
normal RegisterClass instances. Found while I added RegClassByHwMode
to RISC-V and was getting an assertion due to `XLenFVT`/`XLenVecI32VT`
not having an entry for the default mode.