[InstCombine] LLVM ERROR: Instruction Combining did not reach a fixpoint after 1 iterations

Compiler Explorer: https://godbolt.org/z/bjWK99aM7

The following code fails InstCombine with `Instruction Combining did not reach a fixpoint after 1 iterations`:

```llvm
; opt -passes=instcombine -disable-output small.ll
target datalayout = "e-n64"

define void @f(i64 %s) {
entry:
br label %loop

loop:
%a = phi i8 [ 0, %entry ], [ %t, %loop ]
%b = phi i64 [ 0, %entry ], [ %s, %loop ]
%se = sext i8 %a to i64
%or = or i64 %b, %se
%t = trunc i64 %or to i8
%c = icmp eq i64 %s, 0
br i1 %c, label %loop, label %exit

exit:
ret void
}
```

```console
$ opt -passes=instcombine -disable-output small.ll
LLVM ERROR: Instruction Combining did not reach a fixpoint after 1 iterations
```