radeonsi: Add debug option to enable LLVM GlobalISel (v2)

R600_DEBUG=gisel will tell LLVM to use GlobalISel rather than
SelectionDAG for instruction selection.

v2: mareko: move the helper to src/amd/common

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tom Stellard <tstellar@redhat.com>

src/amd/common/ac_llvm_helper.cpp

@@ -171,3 +171,10 @@ void ac_llvm_add_barrier_noop_pass(LLVMPassManagerRef passmgr)
 {
 	llvm::unwrap(passmgr)->add(llvm::createBarrierNoopPass());
 }
+
+void ac_enable_global_isel(LLVMTargetMachineRef tm)
+{
+#if HAVE_LLVM >= 0x0700
+  reinterpret_cast<llvm::TargetMachine*>(tm)->setGlobalISel(true);
+#endif
+}