[Test] Missed opt test for D108910

We can fold loop phis after we've proved that some exit has EC=0
in IndVars.

Patch by Dmitry Makogon!

llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll

@@ -0,0 +1,82 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: -p
+; RUN: opt < %s -indvars -S | FileCheck %s
+; RUN: opt < %s -passes=indvars -S | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+
+declare i1 @foo(i8*, i8*)
+declare i1 @bar()
+declare i1 @baz()
+
+define i1 @kill_backedge_and_phis(i8* align 1 %lhs, i8* align 1 %rhs, i32 %len) {
+; CHECK-LABEL: @kill_backedge_and_phis(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    %length_not_zero = icmp ne i32 %len, 0
+; CHECK-NEXT:    br i1 %length_not_zero, label %loop_preheader, label %exit
+; CHECK:       loop_preheader:
+; CHECK-NEXT:    br label %loop
+; CHECK:       loop:
+; CHECK-NEXT:    %iv = phi i32 [ 0, %loop_preheader ], [ %iv.next, %exiting_3 ]
+; CHECK-NEXT:    %iv.wide = phi i64 [ 0, %loop_preheader ], [ %iv.wide.next, %exiting_3 ]
+; CHECK-NEXT:    %iv.next = add nuw nsw i32 %iv, 1
+; CHECK-NEXT:    %iv.wide.next = add nuw nsw i64 %iv.wide, 1
+; CHECK-NEXT:    %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 %iv
+; CHECK-NEXT:    %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 %iv
+; CHECK-NEXT:    %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)
+; CHECK-NEXT:    br i1 %result, label %exiting_1, label %exit.loopexit
+; CHECK:       exiting_1:
+; CHECK-NEXT:    %iv.wide.is_not_zero = icmp ne i64 %iv.wide, 0
+; CHECK-NEXT:    br i1 false, label %exiting_2, label %exit.loopexit
+; CHECK:       exiting_2:
+; CHECK-NEXT:    %bar_ret = call i1 @bar()
+; CHECK-NEXT:    br i1 %bar_ret, label %exit.loopexit, label %exiting_3
+; CHECK:       exiting_3:
+; CHECK-NEXT:    %baz_ret = call i1 @baz()
+; CHECK-NEXT:    %continue = icmp ne i32 %iv.next, %len
+; CHECK-NEXT:    %or.cond = select i1 %baz_ret, i1 %continue, i1 false
+; CHECK-NEXT:    br i1 %or.cond, label %loop, label %exit.loopexit
+; CHECK:       exit.loopexit:
+; CHECK-NEXT:    %val.ph = phi i1 [ %baz_ret, %exiting_3 ], [ %bar_ret, %exiting_2 ], [ %iv.wide.is_not_zero, %exiting_1 ], [ %result, %loop ]
+; CHECK-NEXT:    br label %exit
+; CHECK:       exit:
+; CHECK-NEXT:    %val = phi i1 [ false, %entry ], [ %val.ph, %exit.loopexit ]
+; CHECK-NEXT:    ret i1 %val
+;
+entry:
+  %length_not_zero = icmp ne i32 %len, 0
+  br i1 %length_not_zero, label %loop_preheader, label %exit
+
+loop_preheader:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %loop_preheader ], [ %iv.next, %latch ]
+  %iv.wide = phi i64 [ 0, %loop_preheader ], [ %iv.wide.next, %latch ]
+  %iv.next = add i32 %iv, 1
+  %iv.wide.next = add i64 %iv.wide, 1
+  %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 %iv
+  %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 %iv
+  %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)
+  br i1 %result, label %exiting_1, label %exit
+
+exiting_1:
+  %iv.wide.is_not_zero = icmp ne i64 %iv.wide, 0
+  br i1 %iv.wide.is_not_zero, label %exiting_2, label %exit
+
+exiting_2:
+  %bar_ret = call i1 @bar()
+  br i1 %bar_ret, label %exit, label %exiting_3
+
+exiting_3:
+  %baz_ret = call i1 @baz()
+  br i1 %baz_ret, label %latch, label %exit
+
+latch:
+  %continue = icmp ne i32 %iv.next, %len
+  br i1 %continue, label %loop, label %exit
+
+exit:
+  %val = phi i1 [ %result, %loop ], [ %iv.wide.is_not_zero, %exiting_1 ],
+  [ %bar_ret, %exiting_2 ], [ %baz_ret, %exiting_3 ],
+  [ %baz_ret, %latch ], [ 0, %entry ]
+  ret i1 %val
+}