PTX: Always use registers for return values, but use .param space for device

PTX: Always use registers for return values, but use .param space for device
     parameters if SM >= 2.0

- Update test cases to be more robust against register allocation changes
- Bump up the number of registers to 128 per type
- Include Python script to re-generate register file with any number of
  registers

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133736 91177308-0d34-0410-b5e6-96231b3b80d8