[llvm-bugs] [Bug 95921] [CodeGen] Eliminate branches using `undef`

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Issue	95921
Summary	[CodeGen] Eliminate branches using `undef`
Labels	llvm:codegen, missed-optimization
Assignees
Reporter	DianQK

    I tried this IR:

```llvm
define { i64, i64 } @src(i64 noundef %arg, i64 noundef %arg1) local_unnamed_addr #0 {
bb:
  switch i64 %arg1, label %bb7 [
    i64 2, label %bb3
    i64 3, label %bb2
 ]

bb2:                                              ; preds = %bb
 br label %bb3

bb7:                                              ; preds = %bb
  %i8 = udiv i64 %arg, %arg1
  %i13 = shl nuw i64 %i8, 1
  br label %bb3

bb3: ; preds = %bb7, %bb2, %bb
  %i = phi i64 [ %i13, %bb7 ], [ %arg, %bb2 ], [ undef, %bb ]
  %i4 = phi i64 [ 1, %bb7 ], [ 1, %bb2 ], [ %arg1, %bb ]
 %i5 = insertvalue { i64, i64 } poison, i64 %i4, 0
  %i6 = insertvalue { i64, i64 } %i5, i64 %i, 1
  ret { i64, i64 } %i6
}
```

The assembly code is:

```asm
src: # @src
  cmp rsi, 2
  je .LBB0_1
  cmp rsi, 3
  je .LBB0_4
  mov rax, rdi
  xor edx, edx
  div rsi
  mov rdi, rax
  add rdi, rax
.LBB0_4: # %bb3
 mov esi, 1
  jmp .LBB0_5
.LBB0_1:
.LBB0_5: # %bb3
  mov rax, rsi
  mov rdx, rdi
  ret
```

We can merge `.LBB0_1` and `.LBB0_5`.

This MIR is:

```llvm
bb.1:
; predecessors: %bb.0
  successors: %bb.5(0x80000000); %bb.5(100.00%)
  liveins: $rsi
  renamable $rdi = IMPLICIT_DEF
```

I think we can remove this BB because we are assigning `undef` to `$rdi`.

GodBolt: https://llvm.godbolt.org/z/joff5fYs9.
From: https://github.com/rust-lang/rust/issues/126585.

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