In some cases (particularly after removing a phi node), we might end up
in a circumstance where it appears statically that we would use a slot
for both a ref and bits. Avoid generating malformed IR in this case.

In the future, this situation could also possibly happen if we walked
through phi nodes and attempted to merge the contents (with great care).

(cherry picked from commit 9e783bb)

Generally we assume parameters can be duplicated without seeing
side-effects. That is not entirely true of mutable globals and
multi-threading.

Refs: #36450
Fixes: #39508
(cherry picked from commit c0f9666)

(cherry picked from commit bf0364b)

…able (#39916)

(cherry picked from commit cc66025)

Fixes backtrace printing to display the number of times a frame is
repeated, if there is a frame that's duplicated several times.

```julia
julia> function foo() foo() end
foo (generic function with 1 method)

julia> foo()
ERROR: StackOverflowError:
Stacktrace:
 [1] foo() (repeats 79984 times)
   @ Main ./REPL[16]:1
```

Fixes #37587.

Co-authored-by: Nathan Daly <nhdaly@gmail.com>
(cherry picked from commit 3276c11)

(cherry picked from commit accce7f)

These ops are not actually legal (they're semantically invalid), but we
temporarily use them to carry information between passes which needs to
then remove them correctly.

Fixes #39508

(cherry picked from commit bd5105e)

Without this, a failure to find a usable `p7zip` will cause an error at
startup.  With this, we will at least attempt to invoke `p7zip` and fail
at runtime, instead of failing at init time.

Backports release 1.6-RC3