[clang] [CIR] Add transform test for cir-flatten-cfg (PR #130861)

[Erich Keane via cfe-commits]

================
@@ -0,0 +1,58 @@
+// RUN: cir-opt %s -cir-flatten-cfg -o - | FileCheck %s
+
+module {
+  cir.func @foo() {
+    cir.scope {
+      %0 = cir.alloca !cir.int<u, 32>, !cir.ptr<!cir.int<u, 32>>, ["a", init] {alignment = 4 : i64}
+      %1 = cir.const #cir.int<4> : !cir.int<u, 32>
+      cir.store %1, %0 : !cir.int<u, 32>, !cir.ptr<!cir.int<u, 32>>
+    }
+    cir.return
+  }
+// CHECK:  cir.func @foo() {
+// CHECK:    cir.br ^bb1
----------------
erichkeane wrote:

> These checks are the same checks that are present in this test in the incubator (except for the fact that we're not abbreviating the int types upstream yet). The test is verifying the flattening pass in isolation, and since the flattening pass doesn't remove unneeded blocks, the output will stay as it is here.

My question was really "should it?" to the "the flattening pass doesn't remove unneeded blocks".  But TBH my understanding of what we should expect out of individual passes, or how passes are designed is lacking.

https://github.com/llvm/llvm-project/pull/130861