[llvm] [clang] [Instcombine] use zext's nneg flag for icmp folding (PR #70845)

[Yingwei Zheng via cfe-commits]

================
@@ -0,0 +1,145 @@
+; RUN: opt < %s --O3 -S | FileCheck %s
----------------
dtcxzyw wrote:

```
define i16 @vecreduce_smin_v2i16(i32 %n, ptr %v) {
entry:
  br label %for.cond

for.cond:
  %p.0 = phi i16 [ 0, %entry ], [ %conv8, %for.inc ]
  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
  %cmp = icmp slt i32 %i.0, %n
  br i1 %cmp, label %for.body, label %for.end

for.body:
  %conv = sext i16 %p.0 to i32
  %idxprom = sext i32 %i.0 to i64
  %arrayidx = getelementptr inbounds i16, ptr %v, i64 %idxprom
  %0 = load i16, ptr %arrayidx, align 2
  %conv1 = sext i16 %0 to i32
  %cmp2 = icmp sgt i32 %conv, %conv1
  br i1 %cmp2, label %cond.true, label %cond.false

cond.true:
  %idxprom4 = sext i32 %i.0 to i64
  %arrayidx5 = getelementptr inbounds i16, ptr %v, i64 %idxprom4
  %1 = load i16, ptr %arrayidx5, align 2
  %conv6 = sext i16 %1 to i32
  br label %cond.end

cond.false:
  %conv7 = sext i16 %p.0 to i32
  br label %cond.end

cond.end:
  %cond = phi i32 [ %conv6, %cond.true ], [ %conv7, %cond.false ]
  %conv8 = trunc i32 %cond to i16
  br label %for.inc

for.inc:
  %inc = add nsw i32 %i.0, 1
  br label %for.cond

for.end:
  ret i16 %p.0
}

define i16 @vecreduce_smax_v2i16(i32 %n, ptr %v) {
entry:
  br label %for.cond

for.cond:
  %p.0 = phi i16 [ 0, %entry ], [ %conv8, %for.inc ]
  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
  %cmp = icmp slt i32 %i.0, %n
  br i1 %cmp, label %for.body, label %for.end

for.body:
  %conv = sext i16 %p.0 to i32
  %idxprom = sext i32 %i.0 to i64
  %arrayidx = getelementptr inbounds i16, ptr %v, i64 %idxprom
  %0 = load i16, ptr %arrayidx, align 2
  %conv1 = sext i16 %0 to i32
  %cmp2 = icmp slt i32 %conv, %conv1
  br i1 %cmp2, label %cond.true, label %cond.false

cond.true:
  %idxprom4 = sext i32 %i.0 to i64
  %arrayidx5 = getelementptr inbounds i16, ptr %v, i64 %idxprom4
  %1 = load i16, ptr %arrayidx5, align 2
  %conv6 = sext i16 %1 to i32
  br label %cond.end

cond.false:
  %conv7 = sext i16 %p.0 to i32
  br label %cond.end

cond.end:
  %cond = phi i32 [ %conv6, %cond.true ], [ %conv7, %cond.false ]
  %conv8 = trunc i32 %cond to i16
  br label %for.inc

for.inc:
  %inc = add nsw i32 %i.0, 1
  br label %for.cond

for.end:
  ret i16 %p.0
}

```

https://github.com/llvm/llvm-project/pull/70845
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