Next stage into switch lowering refactoring

Next stage into switch lowering refactoring
1. Fix some bugs in the jump table lowering threshold
2. Implement much better metric for optimal pivot selection
3. Tune thresholds for different lowering methods
4. Implement shift-and trick for lowering small (<machine word
length) cases with few destinations. Good testcase will follow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35816 91177308-0d34-0410-b5e6-96231b3b80d8