[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 that can be reached with intrinsics with result or operands that aren't legal types.

These can be triggered by mistakenly using a 64-bit mode only intrinsics with a -mtriple=i686. Using report_fatal_error gives a better experience for this mistake in release builds instead of probably crashing.

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

llvm-svn: 352699