Performance optimizations:

- SwitchInst: case values stored separately from Operands List. It allows to make faster access to individual case value numbers or ranges.
- Optimized IntItem, added APInt value caching.
- Optimized IntegersSubsetGeneric: added optimizations for cases when subset is single number or when subset consists from single numbers only.

On my machine these optimizations gave about 4-6% of compile-time improvement.

llvm-svn: 158979