gallivm: fix build on LLVM 12 due to LLVMAddConstantPropagationPass removal

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3465
Cc: 20.1 20.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6531>
1 file changed