all: add LLVM 12 support

Originally based on a PR by @QuLogic, but extended a lot to get all
tests to pass.

transform/maps_test.go

@@ -18,6 +18,7 @@ func TestOptimizeMaps(t *testing.T) {
 		pm := llvm.NewPassManager()
 		defer pm.Dispose()
 		pm.AddDeadStoreEliminationPass()
+		pm.AddAggressiveDCEPass()
 		pm.Run(mod)
 	})
 }

transform/testdata/coroutines.out.ll

@@ -301,13 +301,13 @@ declare i8* @llvm.coro.free(token, i8* nocapture readonly) #0
 ; Function Attrs: nounwind
 declare token @llvm.coro.save(i8*) #2
 
-; Function Attrs: argmemonly nounwind willreturn
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
 declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #3
 
-; Function Attrs: argmemonly nounwind willreturn
+; Function Attrs: argmemonly nofree nosync nounwind willreturn
 declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #3
 
 attributes #0 = { argmemonly nounwind readonly }
 attributes #1 = { nounwind readnone }
 attributes #2 = { nounwind }
-attributes #3 = { argmemonly nounwind willreturn }
+attributes #3 = { argmemonly nofree nosync nounwind willreturn }

transform/transform_test.go

@@ -116,8 +116,9 @@ func filterIrrelevantIRLines(lines []string) []string {
 		if strings.HasPrefix(line, "source_filename = ") {
 			continue
 		}
-		if llvmVersion < 11 && strings.HasPrefix(line, "attributes ") {
+		if llvmVersion < 12 && strings.HasPrefix(line, "attributes ") {
 			// Ignore attribute groups. These may change between LLVM versions.
+			// Right now test outputs are for LLVM 12.
 			continue
 		}
 		out = append(out, line)