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

[Yingwei Zheng via cfe-commits]

================
@@ -0,0 +1,111 @@
+; RUN: opt < %s -O3 -S | FileCheck %s
+; See issue #55013 and PR #70845 for more details.
+; This test comes from the following C program, compiled with clang
+;
+;; short vecreduce_smin_v2i16(int n, short* v)
+;; {
+;;   short p = 0;
+;;   for (int i = 0; i < n; ++i)
+;;     p = p > v[i] ? v[i] : p;
+;;   return p;
+;; }
+;
+;; short vecreduce_smax_v2i16(int n, short* v)
+;; {
+;;   short p = 0;
+;;   for (int i = 0; i < n; ++i)
+;;     p = p < v[i] ? v[i] : p;
+;;   return p;
+;; }
+
+define i16 @vecreduce_smin_v2i16(i32 %n, ptr %v) {
+; CHECK-LABEL: define i16 @vecreduce_smin_v2i16(
+; CHECK:    @llvm.smin.v2i16
+
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %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:                                         ; preds = %for.cond
+  %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:                                        ; preds = %for.body
+  %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:                                       ; preds = %for.body
+  %conv7 = sext i16 %p.0 to i32
+  br label %cond.end
+
+cond.end:                                         ; preds = %cond.false, %cond.true
+  %cond = phi i32 [ %conv6, %cond.true ], [ %conv7, %cond.false ]
+  %conv8 = trunc i32 %cond to i16
+  br label %for.inc
+
+for.inc:                                          ; preds = %cond.end
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret i16 %p.0
+}
+
+define signext i16 @vecreduce_smax_v2i16(i32 noundef %n, ptr noundef %v) {
----------------
dtcxzyw wrote:

```suggestion
define i16 @vecreduce_smax_v2i16(i32 %n, ptr %v) {
```

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