Insert instructions to the entry basic block which initializes the global

Insert instructions to the entry basic block which initializes the global
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156671 91177308-0d34-0410-b5e6-96231b3b80d8