Crash in `CIRIfFlatten` when if clause

[orbiri]

When trying to debug why this reduced snippet from SingleSource fails:

extern void link_error1();

void test4(float x, float y) {
  if ((x == y) || (x != y)) {
  } else
    link_error1();
}

With this crash

Assertion failed: (!NodePtr->isKnownSentinel()), function operator*, file ilist_iterator.h, line 168.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.      Program arguments: /Users/orbiri/cir/llvm-project/build/Release/bin/cir-opt --mlir-print-ir-after-failure --cir-to-llvm --reconcile-unrealized-casts /tmp/test.cir -o /tmp/test.mlir
 #0 0x0000000107267a28 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/Users/orbiri/cir/llvm-project/build/Release/bin/cir-opt+0x101418a28)
 #1 0x0000000107265929 llvm::sys::RunSignalHandlers() (/Users/orbiri/cir/llvm-project/build/Release/bin/cir-opt+0x101416929)
 #2 0x0000000107268096 SignalHandler(int) (/Users/orbiri/cir/llvm-project/build/Release/bin/cir-opt+0x101419096)
 #3 0x00007ff812efce9d (/usr/lib/system/libsystem_platform.dylib+0x7ff80047fe9d)
 #4 0x00007ff7ba0af3f0 
 #5 0x00007ff812de8b19 (/usr/lib/system/libsystem_c.dylib+0x7ff80036bb19)
 #6 0x00007ff812de7ddc (/usr/lib/system/libsystem_c.dylib+0x7ff80036addc)
 #7 0x00000001074d1d93 (anonymous namespace)::CIRIfFlattening::matchAndRewrite(mlir::cir::IfOp, mlir::PatternRewriter&) const (.cold.14) (/Users/orbiri/cir/llvm-project/build/Release/bin/cir-opt+0x101682d93)

I reduced the problem to

#true = #cir.bool<true> : !cir.bool

cir.func @test4() {
    %6 = cir.const #true
    cir.if %6 {
    } else {
    }
    cir.return
  }

While the following snippet do pass

#true = #cir.bool<true> : !cir.bool

cir.func @test4() {
    %6 = cir.const #true
    cir.if %6 {
          %5 = cir.const #true

    } else {
    }
    cir.return
  }

---

I am now building with debug and will try to understand the cause for this issue :)

[orbiri]

I have a working solution that also upholds to the guarantee of terminating cir.scopes. Will push later today after verifying a rebase passes all tests

[orbiri]

Will be solved by #1097