[LegalizeTypes] Use report_fatal_error instead of llvm_unreachable in the default...

[LegalizeTypes] Use report_fatal_error instead of llvm_unreachable in the default case of some type legalization handlers.

These can be triggered by in various ways when intrinsics are used wrong or a target doesn't correctly
not support something. Using a fatal error prevents strange behavior
like infinite loops.

We already do this for some of the vector type legalization handles.