[SimpleLoopUnswitch] Add a test case exposing a bug

[DaniilSuchkov]

Summary

This test exposes a bug in SimpleLoopUnswitch that leads to a crash on assert(SuccessorsCount > 1 && "Cannot unswitch a condition without multiple distinct successors!"); when SimpleLoopUnswitch considers unswitching of a loop by a switch with one successor.

Fix will be submitted soon.

[DaniilSuchkov]

@skatkov, could you please submit this patch? I don't have permissions to submit patches yet.

llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-redundant-switch.ll

+; REQUIRES: asserts
+; XFAIL: *
+; RUN: opt -passes='unswitch<nontrivial>' -disable-output -S < %s
+; RUN: opt -simple-loop-unswitch -enable-nontrivial-unswitch -disable-output -S < %s
+
+; This loop shouldn't trigger asserts in SimpleLoopUnswitch.
+define void @test_redundant_switch(i1* %ptr, i32 %cond) {
+entry:
+  br label %loop_begin
+
+loop_begin:
+  switch i32 %cond, label %loop_body [
+      i32 0, label %loop_body
+  ]
+
+loop_body:
+  br label %loop_latch
+
+loop_latch:
+  %v = load i1, i1* %ptr
+  br i1 %v, label %loop_begin, label %loop_exit
+
+loop_exit:
+  ret void
+}