[Polly][OpTree] Fix mid-processing change of access kind.

Operand tree forwarding can cause the change of an access kind; in
particular change from a scalar kind to an array kind if the scalar
dependency is not necessary. Such an access cannot and doesn't need to
be forwarded anymore.

Fixes llvm.org/PR48034

polly/lib/Transform/ForwardOpTree.cpp

@@ -979,17 +979,14 @@ class ForwardOpTreeImpl : ZoneAlgorithm {
 
       // Because we are modifying the MemoryAccess list, collect them first to
       // avoid iterator invalidation.
-      SmallVector<MemoryAccess *, 16> Accs;
-      for (MemoryAccess *RA : Stmt) {
+      SmallVector<MemoryAccess *, 16> Accs(Stmt.begin(), Stmt.end());
+
+      for (MemoryAccess *RA : Accs) {
         if (!RA->isRead())
           continue;
         if (!RA->isLatestScalarKind())
           continue;
 
-        Accs.push_back(RA);
-      }
-
-      for (MemoryAccess *RA : Accs) {
         if (tryForwardTree(RA)) {
           Modified = true;
           StmtModified = true;

polly/test/ForwardOpTree/changed-kind.ll

@@ -0,0 +1,54 @@
+; RUN: opt %loadPolly -polly-optree -analyze < %s | FileCheck %s -match-full-lines
+
+; In the code below, %0 is known to be equal to the content of @c (constant 0).
+; Thus, in order to save a scalar dependency, forward-optree replaces
+; the use of %0 in Stmt_lor_end93 by a load from @c by changing the
+; access find from a scalar access to a array accesses.
+; llvm.org/PR48034 decribes a crash caused by the mid-processing change.
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@c = external dso_local global i64, align 8
+
+define void @func()  {
+entry:
+  br label %lor.end
+
+while.cond.loopexit:
+  %conv102.le = trunc i64 %xor101 to i8
+  ret void
+
+lor.end:
+  %tobool72.not = icmp eq i64 0, 0
+  br i1 %tobool72.not, label %lor.rhs87, label %lor.end.thread
+
+lor.end.thread:
+  br label %lor.rhs87
+
+lor.rhs87:
+  %0 = phi i64 [ 0, %lor.end.thread ], [ 0, %lor.end ]
+  store i64 %0, i64* @c, align 8
+  %neg79 = xor i64 %0, -1
+  br label %lor.end93
+
+lor.end93:
+  %tobool93 = icmp ne i64 undef, 0
+  %conv95 = zext i1 %tobool93 to i64
+  %and100 = and i64 %conv95, undef
+  %xor101 = xor i64 %and100, %neg79
+  %xor103 = xor i64 %0, %conv95
+  br label %while.cond.loopexit
+}
+
+
+; CHECK: Statistics {
+; CHECK:     Reloads: 1
+; CHECK: }
+
+; CHECK: After statements {
+; CHECK:     Stmt_lor_end93
+; CHECK-NEXT:        ReadAccess :=       [Reduction Type: NONE] [Scalar: 1]
+; CHECK-NEXT:            { Stmt_lor_end93[] -> MemRef3[] };
+; CHECK-NEXT:       new: { Stmt_lor_end93[] -> MemRef_c[0] };
+; CHECK: }