Performance optimizations:

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158979 91177308-0d34-0410-b5e6-96231b3b80d8