[SelectionDAG] Replace llvm_unreachable at the end of getCopyFromParts with a report_fatal_error.

Based on PR41748, not all cases are handled in this function.

llvm_unreachable is treated as an optimization hint than can prune code paths
in a release build. This causes weird behavior when PR41748 is encountered on a
release build. It appears to generate an fp_round instruction from the floating
point code.

Making this a report_fatal_error prevents incorrect optimization of the code
and will instead generate a message to file a bug report.

llvm-svn: 360008