[SimplifyCFG] Transform for redirecting phis between unmergeable BB and SuccBB (#67275)

This patch extends function TryToSimplifyUncondBranchFromEmptyBlock to
handle the similar cases below.

```llvm
define i8 @src(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %case012
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %end
  ]

unreachable:
  unreachable

case1:
  br label %case012

case2:
  br label %case012

case012:
  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  br label %end

end:
  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
  ret i8 %phi2
}
```
The phis here should be merged into one phi, so that we can better
optimize it:

```llvm
define i8 @tgt(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %end
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %case3
  ]

unreachable:
  unreachable

case1:
  br label %end

case2:
  br label %end

case3:
  br label %end

end:
  %phi = phi i8 [ 4, %case3 ], [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  ret i8 %phi
}
```
Proof:
[normal](https://alive2.llvm.org/ce/z/vAWi88)
[multiple stages](https://alive2.llvm.org/ce/z/DDBQqp)
[multiple stages 2](https://alive2.llvm.org/ce/z/nGkeqN)
[multiple phi combinations](https://alive2.llvm.org/ce/z/VQeEdp)

And lookup table optimization should convert it into add %arg 1.
This patch just match similar CFG structure and merge the phis in
different cases.

Maybe such transform can be applied to other situations besides switch,
but I'm not sure whether it's better than not merging. Therefore, I only
try it in switch,

Related issue:
#63876

[Migrated](https://reviews.llvm.org/D155940)

0 files changed, 0 insertions, 0 deletions