[LoopUnroll] Add test for partial unrolling again non-latch exit (NFC)

This test case would get miscompiled by the current version of
D102982, because unrolling does not respect the PreserveCondBr
flag for partial unrolling.

llvm/test/Transforms/LoopUnroll/partial-unroll-non-latch-exit.ll

@@ -0,0 +1,48 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -loop-unroll -unroll-allow-partial %s | FileCheck %s
+
+; This is a variant on full-unroll-non-latch-exit.ll for partial unrolling.
+; This test is primarily interested in making sure that latches are not
+; folded incorrectly, not that a transform occurs.
+
+define i1 @test(i64* %a1, i64* %a2) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[START:%.*]] ], [ [[IV_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[IV]], 24
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[EXIT:%.*]], label [[LATCH]]
+; CHECK:       latch:
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds i64, i64* [[A1:%.*]], i64 [[IV]]
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr inbounds i64, i64* [[A2:%.*]], i64 [[IV]]
+; CHECK-NEXT:    [[LOAD1:%.*]] = load i64, i64* [[GEP1]], align 8
+; CHECK-NEXT:    [[LOAD2:%.*]] = load i64, i64* [[GEP2]], align 8
+; CHECK-NEXT:    [[EXITCOND2:%.*]] = icmp eq i64 [[LOAD1]], [[LOAD2]]
+; CHECK-NEXT:    br i1 [[EXITCOND2]], label [[LOOP]], label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[EXIT_VAL:%.*]] = phi i1 [ false, [[LATCH]] ], [ true, [[LOOP]] ]
+; CHECK-NEXT:    ret i1 [[EXIT_VAL]]
+;
+start:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %start ], [ %iv.next, %latch ]
+  %exitcond = icmp eq i64 %iv, 24
+  br i1 %exitcond, label %exit, label %latch
+
+latch:
+  %iv.next = add nuw nsw i64 %iv, 1
+  %gep1 = getelementptr inbounds i64, i64* %a1, i64 %iv
+  %gep2 = getelementptr inbounds i64, i64* %a2, i64 %iv
+  %load1 = load i64, i64* %gep1, align 8
+  %load2 = load i64, i64* %gep2, align 8
+  %exitcond2 = icmp eq i64 %load1, %load2
+  br i1 %exitcond2, label %loop, label %exit
+
+exit:
+  %exit.val = phi i1 [ false, %latch ], [ true, %loop ]
+  ret i1 %exit.val
+}