ac: add if/loop build helpers

These have been ported over from radeonsi.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 42627dab)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/amd/common/ac_llvm_build.c
	src/amd/common/ac_llvm_build.h