[InstCombine] Regard `zext nneg` as `sext` when folding `add(zext neg(add))`

[dianqk]

The following transformation is legal after adding nneg:

define i32 @src(i8 %v) {
  %switch.tableidx = add nsw i8 %v, 1
  %switch.idx.cast = zext nneg i8 %switch.tableidx to i32
  %switch.offset = add nsw i32 %switch.idx.cast, -1
  ret i32 %switch.offset
}

define i32 @tgt(i8 %v) {
  %ext = sext i8 %v to i32
  ret i32 %ext
}

Alive2: https://alive2.llvm.org/ce/z/2iCmDs
From: rust-lang/rust#106459 (comment)

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Quentin Dian (DianQK)

The following transformation is legal after adding `nneg`:

define i32 @<!-- -->src(i8 %v) {
  %switch.tableidx = add nsw i8 %v, 1
  %switch.idx.cast = zext nneg i8 %switch.tableidx to i32
  %switch.offset = add nsw i32 %switch.idx.cast, -1
  ret i32 %switch.offset
}

define i32 @<!-- -->tgt(i8 %v) {
  %ext = sext i8 %v to i32
  ret i32 %ext
}

Alive2: https://alive2.llvm.org/ce/z/2iCmDs
From: rust-lang/rust#106459 (comment)

[XChy]

Should be easy to fix in:

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

Line 830 in efb8cc5

// (sext (X +nsw NarrowC)) + C --> (sext X) + (sext(NarrowC) + C)

[ZelinMa557]

Can I work on this issue? I'm glad to try it :)