[llvm] [SCEV] Add special handling for phi when BranchInst's edge doesn't dominate use in phi (PR #163146)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Actually, I'm not understanding the pattern.  Can you write it in Alive2?  I got as far as the following (https://alive2.llvm.org/ce/z/7WAcTS):

```
define i32 @src(i32 %a, i1 %p) {
entry:
  %b = and i32 %a, 31
  %c = icmp eq i32 %b, 0
  br i1 %c, label %cmp_true, label %cmp_false

cmp_false:
  br i1 %p, label %cmp_true, label %merge

cmp_true:
  br label %merge

merge:
  %d = phi i32 [32, %cmp_true], [%b, %cmp_false]
  ret i32 %d
}

define i32 @tgt(i32 %a, i1 %p) {
  %b = and i32 %a, 31
  %add = add i32 %a, 1
  %max = call i32 @llvm.umax.i32(i32 %add, i32 1)
  ret i32 %max
}
```

https://github.com/llvm/llvm-project/pull/163146