Factor out a splitSwitchCase() function so that it can be reused.

This is in preparation for a fix to llvm.org/PR22262. One of the ideas
here is to first find a good jump table range first and then split
before and after it. Thereby, we don't need to use the
split-based-on-density heuristic at all, which can make the "binary
tree" deteriorate in various cases.

Also some minor cleanups.

No functional changes.

llvm-svn: 226551