[LICM] Add test for PR51248.

Test for #51248. LICM introduces an unused load in a writeonly function.

llvm/test/Transforms/LICM/scalar-promote.ll

@@ -599,6 +599,65 @@ Out:
 
 }
 
+@glb = external global i8, align 1
+
+; Test case for PR51248.
+define void @test_sink_store_only(i8 %var, i64 %start) writeonly {
+; CHECK-LABEL: @test_sink_store_only(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i8 [[VAR:%.*]], 3
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i8 [[DIV]], 0
+; CHECK-NEXT:    [[GLB_PROMOTED:%.*]] = load i8, i8* @glb, align 1
+; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
+; CHECK:       loop.header:
+; CHECK-NEXT:    [[MERGE1:%.*]] = phi i8 [ [[GLB_PROMOTED]], [[ENTRY:%.*]] ], [ [[MERGE:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[I:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ [[ADD:%.*]], [[LOOP_LATCH]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i64 [[I]], 4
+; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       loop.body:
+; CHECK-NEXT:    br i1 [[CMP2]], label [[COND_TRUE:%.*]], label [[COND_FALSE:%.*]]
+; CHECK:       cond.true:
+; CHECK-NEXT:    br label [[LOOP_LATCH]]
+; CHECK:       cond.false:
+; CHECK-NEXT:    br label [[LOOP_LATCH]]
+; CHECK:       loop.latch:
+; CHECK-NEXT:    [[MERGE]] = phi i8 [ [[DIV]], [[COND_TRUE]] ], [ 0, [[COND_FALSE]] ]
+; CHECK-NEXT:    [[ADD]] = add i64 [[I]], 4
+; CHECK-NEXT:    br label [[LOOP_HEADER]]
+; CHECK:       for.end:
+; CHECK-NEXT:    [[MERGE1_LCSSA:%.*]] = phi i8 [ [[MERGE1]], [[LOOP_HEADER]] ]
+; CHECK-NEXT:    store i8 [[MERGE1_LCSSA]], i8* @glb, align 1
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop.header
+
+loop.header:
+  %i = phi i64 [ 0, %entry ], [ %add, %loop.latch ]
+  %cmp = icmp ult i64 %i, 4
+  br i1 %cmp, label %loop.body, label %for.end
+
+loop.body:
+  %div = sdiv i8 %var, 3
+  %cmp2 = icmp slt i8 %div, 0
+  br i1 %cmp2, label %cond.true, label %cond.false
+
+cond.true:
+  br label %loop.latch
+
+cond.false:
+  br label %loop.latch
+
+loop.latch:
+  %merge = phi i8 [ %div, %cond.true ], [ 0, %cond.false ]
+  store i8 %merge, i8* @glb, align 1
+  %add = add i64 %i, 4
+  br label %loop.header
+
+for.end:
+  ret void
+}
+
 !0 = !{!4, !4, i64 0}
 !1 = !{!"omnipotent char", !2}
 !2 = !{!"Simple C/C++ TBAA"}