[PPCGCodeGeneration] Allow intrinsics within kernels.

- In D33414, if any function call was found within a kernel, we would bail out.

- This is an over-approximation. This patch changes this by allowing the
  `llvm.sqrt.*` family of intrinsics.

- This introduces an additional step when creating a separate llvm::Module
  for a kernel (GPUModule). We now copy function declarations from the
  original module to new module.

- We also populate IslNodeBuilder::ValueMap so it replaces the function
  references to the old module to the ones in the new module
  (GPUModule).

Differential Revision: https://reviews.llvm.org/D34145

llvm-svn: 306284