[llvm] [InstCombine] Allow freezing multiple out-of-loop values (PR #155638)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> Can you please add a test where the two start values come from the same predecessor (e.g. dummy br with both successors equal)? You need to be careful about that because the phi needs to have identical values for the same predecessor.
> 
>> Perhaps I'm misunderstanding, but that's not legal IR? https://godbolt.org/z/8MYM1Yf5b

Apologies I think I understand your concern now. In this example: https://godbolt.org/z/W5e3rfx1E we want to avoid:
```
define void @fold_phi_multiple_identical_start_values(i1 %c, i32 %init, i32 %n) {
entry:
  %init.fr.0 = freeze i32 %init
  %init.fr.1 = freeze i32 %init
  br i1 %c, label %loop, label %loop

loop:
  %i = phi i32 [ %init.fr.0, %entry ], [ %init.fr.1, %entry ], [ %i.next, %loop ]
  %i.next = add nsw nuw i32 %i, 1
  %cond = icmp eq i32 %i.next, %n
  br i1 %cond, label %loop, label %exit

exit:
  ret void
}
```

the final IR is correct with `%init.fr` for both `%entry` incoming values for this.

> By the way, this change isn't actually what I had in mind... I'd expect multiple starting values to get canonicalized away by loop simplify form.

You're right, if the multiple starting values are well-defined the freeze can be removed with a combination of loop-simplify and instcombine: https://godbolt.org/z/dh7jq5qov

> I think the more interesting extension here is the case where you need to freeze both the start value and the step value. So something like {start,+,step} where both values are not known non-poison.

Seems this is already handled: https://godbolt.org/z/9oa4YvnYz

Not sure this patch has any value. It also doesn't help the test I added in #157112 either as that's not a recurrence.

https://github.com/llvm/llvm-project/pull/155638