Switch lowering: use profile info to build weight-balanced binary search trees

This will cause hot nodes to appear closer to the root.

The literature says building the tree like this makes it a near-optimal (in
terms of search time given key frequencies) binary search tree. In LLVM's case,
we can do up to 3 comparisons in each leaf node, so it might be better to opt
for lower tree height in some cases; that's something to look into in the
future.

Differential Revision: http://reviews.llvm.org/D9318

llvm-svn: 236192