Strengthen the "non-constant stride must dominate loop preheader" check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64703 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/LoopStrengthReduce/pr3399.ll

@@ -0,0 +1,32 @@
+; RUN: llvm-as < %s | opt -loop-reduce | llvm-dis
+; PR3399
+
+@g_53 = external global i32		; <i32*> [#uses=1]
+
+define i32 @foo() nounwind {
+bb5.thread:
+	br label %bb
+
+bb:		; preds = %bb5, %bb5.thread
+	%indvar = phi i32 [ 0, %bb5.thread ], [ %indvar.next, %bb5 ]		; <i32> [#uses=2]
+	br i1 false, label %bb5, label %bb1
+
+bb1:		; preds = %bb
+	%l_2.0.reg2mem.0 = sub i32 0, %indvar		; <i32> [#uses=1]
+	%0 = volatile load i32* @g_53, align 4		; <i32> [#uses=1]
+	%1 = trunc i32 %l_2.0.reg2mem.0 to i16		; <i16> [#uses=1]
+	%2 = trunc i32 %0 to i16		; <i16> [#uses=1]
+	%3 = mul i16 %2, %1		; <i16> [#uses=1]
+	%4 = icmp eq i16 %3, 0		; <i1> [#uses=1]
+	br i1 %4, label %bb7, label %bb2
+
+bb2:		; preds = %bb2, %bb1
+	br label %bb2
+
+bb5:		; preds = %bb
+	%indvar.next = add i32 %indvar, 1		; <i32> [#uses=1]
+	br label %bb
+
+bb7:		; preds = %bb1
+	ret i32 1
+}