[llvm] [CGSCC] Fix compile time blowup with large RefSCCs (PR #94815)

[Lei Wang via llvm-commits]

wlei-llvm wrote:

> @wlei-llvm yes a reduced repro would be helpful, I can't figure out what's wrong with just the assert

Hi @aeubanks , here is the reduced repro:

cmd:
```
opt -disable-output -passes="cgscc(devirt<4>(inline,openmp-opt-cgscc,function<>(simplifycfg<>)))" < reduced.ll
```

reduced.ll:
```
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-redhat-linux-gnu"
define internal fastcc void @foo() #0 personality ptr null {
  br label %1
1:                                                ; preds = %2, %0
  ret void
2:                                                ; No predecessors!
  call fastcc void @bar()
  br label %1
}
define fastcc void @baz() personality ptr null {
  switch i64 0, label %3 [
    i64 0, label %3
    i64 1, label %3
    i64 2, label %3
    i64 3, label %4
    i64 4, label %3
    i64 5, label %3
    i64 6, label %3
    i64 7, label %3
    i64 8, label %3
    i64 9, label %3
    i64 10, label %3
    i64 11, label %3
    i64 12, label %3
    i64 13, label %3
    i64 14, label %3
    i64 15, label %3
    i64 16, label %3
  ]
1:                                                ; No predecessors!
  %2 = landingpad { ptr, i32 }
          cleanup
  call fastcc void @foo()
  resume { ptr, i32 } zeroinitializer
3:                                                ; preds = %4, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0, %0
  ret void
4:                                                ; preds = %0
  call fastcc void @baz()
  br label %3
}
define fastcc void @bar() personality ptr null {
  call fastcc void @baz()
  ret void
}
; uselistorder directives
uselistorder ptr null, { 3, 4, 0, 5, 6, 1, 7, 8, 2 }
attributes #0 = { "target-cpu"="haswell" }
!llvm.module.flags = !{!0}
!0 = !{i32 7, !"openmp", i32 50}
```
Please take a look, thanks! 

https://github.com/llvm/llvm-project/pull/94815