[SelectionDAG] Use report_fatal_error instead of llvm_unreachable in some code that...

[SelectionDAG] Use report_fatal_error instead of llvm_unreachable in some code that can be reached if targets don't configure things correctly.

For example, this is currently reachable by X86 if you use a masked store intrinsic with a v1iX type.

Using a fatal error seems like a better user experience if someone were to encounter this on a release build. There are several other similar places that have been converted from unreachable to fatal error previously.

llvm-svn: 318379