Flow blocks are generated code that don't really correspond to any
location in the source, so principally they should have empty DebugLocs.
Practically, setting these debug locs leads to redundant is_stmts being
generated after #108251, causing stepping test failures in the ROCm GDB
test suite.

Fixes SWDEV-502134

There are a surprising number of codegen changes from this.

I tried to fix it in #124051 but failed to do so. This PR adds the test
and
marks it as xfail.

This PR removes tests with `br i1 undef` under
`llvm/tests/Transforms/ObjCARC, Reassociate, SCCP, SLPVectorizer...`.
After this PR, I'll continue to fix tests under `llvm/tests/CodeGen`,
which has more UB tests than `llvm/tests/Transforms`.

This reverts commit be40c723ce2b7bf2690d22039d74d21b2bd5b7cf.

Currently `StructurizeCFG` drops branch_weight metadata .
This metadata can be generated from user annotations in the source code
like:

```cpp
if (...) [[likely]] {
}
```

[FixIrreducible] Use CycleInfo instead of a custom SCC traversal

1. CycleInfo efficiently locates all cycles in a single pass, while the
SCC is
   repeated inside every natural loop.

2. CycleInfo provides a hierarchy of irreducible cycles, and the new
implementation transforms each cycle in this hierarchy separately
instead of
reducing an entire irreducible SCC in a single step. This reduces the
number
of control-flow paths that pass through the header of each newly created
loop. This is evidenced by the reduced number of predecessors on the
"guard"
blocks in the lit tests, and fewer operands on the corresponding PHI
nodes.

3. When an entry of an irreducible cycle is the header of a child
natural loop,
the original implementation destroyed that loop. This is now preserved,
   since the incoming edges on non-header entries are not touched.

4. In the new implementation, if an irreducible cycle is a superset of a
natural
loop with the same header, then that natural loop is destroyed and
replaced
   by the newly created loop.