[llvm] [InstCombine] Fix fail to fold (A >> C1) Pred C2 if shr is used multple times #83430 (PR #83563)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> @dtcxyzw,
> 
> ```c++
> if (IsAShr && matchSelectPattern(Shr->user_back(), Lhs, Rhs).Flavor == SPF_UNKNOWN) {
> ```
> 
> After I made some code changes, this line now triggers three times. However, it only returns SPF_SMIN twice. On the third call, Shr->user_back() seems to be behaving unexpectedly, as shown by the following output:
> 
> ```
> (lldb) expr Shr->user_back()->dump()
>  %cmp4.i = icmp slt i64 %shr, 348732
> ```
> 
> Can you help me understand why this discrepancy might be happening?

`%spec.select.i = select i1 %switch.i, i64 %shr, i64 348731` seems to be eliminated. You can pass `-debug-only=instcombine` to opt for more details :)

https://github.com/llvm/llvm-project/pull/83563