pointer register. 


This is the first of the series of patches which clean up the way global pointer
register is used. The patches will make the following improvements:

- Make $gp an allocatable temporary register rather than reserving it.
- Use a virtual register as the global pointer register and let the register
  allocator decide which register to assign to it or whether spill/reloads are
  needed.
- Make sure $gp is valid at the entry of a called function, which is necessary
  for functions using lazy binding.
- Remove the need for emitting .cprestore and .cpload directives.

llvm-svn: 156671