[LAA] Add a test case to show incorrect dependency classification (NFC).

Currently the loop access analysis classifies this loop as unsafe to vectorize
because the memory dependencies are 'ForwardButPreventsForwarding'. However,
the access pattern is 'write-after-read' with no subsequent read accessing
the written memory locations. I can't see how store-to-load forwarding is
applicable here.

void vectorizable_Read_Write(int *A) {
  for (unsigned i = 1022; i >= 0; i--)
    A[i+1] = A[i] + 1;
}

Author: labrinea

llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll

@@ -0,0 +1,40 @@
+; RUN: opt -passes='print<access-info>' -disable-output  < %s 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
+
+; FIXME: This should be vectorizable
+
+; void vectorizable_Read_Write(int *A) {
+;  for (unsigned i = 1022; i >= 0; i--)
+;    A[i+1] = A[i] + 1;
+; }
+
+; CHECK: function 'vectorizable_Read_Write':
+; CHECK-NEXT:   for.body:
+; CHECK-NEXT:     Report: unsafe dependent memory operations in loop
+; CHECK-NEXT:     Forward loop carried data dependence that prevents store-to-load forwarding.
+; CHECK-NEXT:     Dependences:
+; CHECK-NEXT:       ForwardButPreventsForwarding:
+; CHECK-NEXT:           %0 = load i32, ptr %arrayidx, align 4 ->
+; CHECK-NEXT:           store i32 %add, ptr %gep, align 4
+
+define void @vectorizable_Read_Write(ptr nocapture %A) {
+entry:
+  %invariant.gep = getelementptr i32, ptr %A, i64 1
+  br label %for.body
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %indvars.iv = phi i64 [ 1022, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv
+  %0 = load i32, ptr %arrayidx, align 4
+  %add = add nsw i32 %0, 1
+  %gep = getelementptr i32, ptr %invariant.gep, i64 %indvars.iv
+  store i32 %add, ptr %gep, align 4
+  %indvars.iv.next = add nsw i64 %indvars.iv, -1
+  %cmp.not = icmp eq i64 %indvars.iv, 0
+  br i1 %cmp.not, label %for.cond.cleanup, label %for.body
+}
+