Copied from old Shader::GetFunction().

Bug 22533227

Change-Id: I587a4112bc97f75c010b6f572f8f4388d5af9225
Reviewed-on: https://swiftshader-review.googlesource.com/3734Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>