[llvm-bugs] [Bug 134908] InstCombine introduces unnecessary `and 1` when folding `trunc nuw`+`zext`

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Issue	134908
Summary	InstCombine introduces unnecessary `and 1` when folding `trunc nuw`+`zext`
Labels	new issue
Assignees
Reporter	scottmcm

    Rust in which we spotted this originally: <https://rust.godbolt.org/z/Knr5jdEY5>, cc https://github.com/rust-lang/rust/pull/139098#discussion_r2032140281

Take this input IR:
```llvm
define noundef range(i8 0, 14) i8 @match0(i8 noundef range(i8 0, 3) %0) unnamed_addr {
start:
  %1 = icmp eq i8 %0, 2
  %_2 = select i1 %1, i64 1, i64 0
  %2 = trunc nuw i64 %_2 to i1
  br i1 %2, label %bb2, label %bb3

bb2:                                              ; preds = %start
  br label %bb4

bb3:                                              ; preds = %start
  %b = trunc nuw i8 %0 to i1
  %3 = zext i1 %b to i8
  br label %bb4

bb4:                                              ; preds = %bb2, %bb3
  %_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %3, %bb3 ]
  ret i8 %_0.sroa.0.0
}
```

Today <https://llvm.godbolt.org/z/vnfGGxfbr>, `--passes=instcombine` transforms that to
```llvm
source_filename = "/app/example.ll"

define noundef range(i8 0, 14) i8 @match0(i8 noundef range(i8 0, 3) %0) unnamed_addr {
  %1 = icmp eq i8 %0, 2
  br i1 %1, label %bb2, label %bb3

bb2:                                              ; preds = %start
  br label %bb4

bb3:                                              ; preds = %start
  %2 = and i8 %0, 1 ; 😢
  br label %bb4

bb4:                                              ; preds = %bb3, %bb2
  %_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %2, %bb3 ]
  ret i8 %_0.sroa.0.0
}
```

However it should not be adding that `and i8 %0, 1` -- the `trunc nuw` means it ought to know it doesn't need to do that.

It should instead be emitting <https://alive2.llvm.org/ce/z/oN3dig>
```llvm
define noundef range(i8 0, 14) i8 @tgt(i8 noundef range(i8 0, 3) %0) unnamed_addr {
start:
  %1 = icmp eq i8 %0, 2
  br i1 %1, label %bb2, label %bb3

bb2:                                              ; preds = %start
  br label %bb4

bb3:                                              ; preds = %start
  br label %bb4

bb4:                                              ; preds = %bb3, %bb2
  %_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %0, %bb3 ]
  ret i8 %_0.sroa.0.0
}
```

Note that the correctness for this doesn't depend on the range information; removing that from the parameter it still validates <https://alive2.llvm.org/ce/z/_ed2Ch>.

(If you run a full -O2 or -O3 then CVP will get rid of the `and`, but that's not run in -O1.  And InstCombine has the information it needs to do it correctly anyway, AFAICT, so it just should.)

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