gallium: add opaque pointers shim for LLVM < 8.0

gallium: add opaque pointers shim for LLVM < 8.0

LLVM is transitioning to "opaque pointers", and as such deprecates
LLVMBuildGEP, LLVMBuildCall, LLVMBuildLoad, replacing them with
LLVMBuildGEP2, LLVMBuildCall2, LLVMBuildLoad2 respectivelly.
These new functions were added in LLVM 8.0; so for LLVM before 8.0 we
simply forward to the non-opaque-pointer variants.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15893>