[TargetLowering] Remove weird use of MVT::isVoid in an assert. (#101436)

At the time this was written there were no vector types in MVT. The
order was:
-scalar integer types
-scalar FP types
-isVoid

I believe this isVoid check was to catch walking off the end of the
scalar FP types. While the isInteger()==isInteger caught walking off the
end of scalar integer types.

These days we have:
-scalar integer types
-scalar FP types
-fixed vector integer types
-fixed vector FP types
-scalable vector integer types
-scalable vector FP types.
-Glue
-isVoid

So checking isVoid doesn't detect what it used to. I've changed it to
check isFloatingPoint() == isFloatingPoint() instead.