Prevent binary-tree deterioration in sparse switch statements.

This addresses part of llvm.org/PR22262. Specifically, it prevents
considering the densities of sub-ranges that have fewer than
TLI.getMinimumJumpTableEntries() elements. Those densities won't help
jump tables.

This is not a complete solution but works around the most pressing
issue.

Review: http://reviews.llvm.org/D7070
llvm-svn: 226600