[SimplifyCFG] Simplify uncond br with icmp & select (#165580)

Previously, SimplifyCFG only simplified unconditional branches when they
met a pattern (`swicth` -> `icmp` -> `br` -> `phi`) as follows:
```LLVM
   switch i8 %A, label %DEFAULT [ i8 1, label %end    i8 2, label %end ]
DEFAULT:
   %tmp = icmp eq i8 %A, 92
   br label %end
end:
   ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
```

This PR supports a new and more generic pattern (`switch` -> `icmp` ->
`select` -> `br` -> `phi` ) to simplify unconditional branches as
follows:
```LLVM
; BEFORE
case1:
  switch i32 %x, label %DEFAULT [ 
    i32 0, label %end    
    i32 1, label %case2 
  ]
case2:
  br label %end
DEFAULT:
  %tmp = icmp eq i32 %x, 2
  %val = select i1 %tmp, i32 V3, i32 V4
  br label %end
end:
  ... = phi i32 [ V1, %case1 ], [ V2, %case2 ], [ %val, %DEFAULT ]
```

We prefer to split the edge to 'end' so that there are TWO entries of
V3/V4 to the PHI, merging the icmp & select into the switch, as follows:
```LLVM
; AFTER
case1:
  switch i32 %x, label %DEFAULT [
    i32 0, label %end
    i32 1, label %case2
    i32 2, label %case3
  ]
case2:
  br label %end
case3:
  br label %end
DEFAULT:
  br label %end
end:
  ... = phi i32 [ V1, %case1 ], [ V2, %case2 ], [ V3, %case3 ], [ V4, %DEFAULT]
```

Alive2 Proof: https://alive2.llvm.org/ce/z/jYHM4f
Promising Optimization Impact:
https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3006

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