swr: Fix build with llvm-9.0.

swr: Fix build with llvm-9.0.

Fix build error after llvm-9.0svn r352827 ("[opaque pointer types] Add a
FunctionCallee wrapper type, and use it.").

In file included from ./rasterizer/jitter/builder.h:158:0,
                 from swr_shader.cpp:35:
./rasterizer/jitter/gen_builder_meta.hpp: In member function ‘llvm::Value* SwrJit::Builder::VGATHERPD(llvm::Value*, llvm::Value*, llvm::Value*, llvm::Value*, llvm::Value*, const llvm:
:Twine&)’:
./rasterizer/jitter/gen_builder_meta.hpp:51:117: error: no matching function for call to ‘cast(llvm::FunctionCallee)’
     Function* pFunc = cast<Function>(JM()->mpCurrentModule->getOrInsertFunction("meta.intrinsic.VGATHERPD", pFuncTy));
                                                                                                                     ^

Suggested-by: Philip Meulengracht <the_meulengracht@hotmail.com>
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Alok Hota <alok.hota@intel.com>