[ConstProp] Remove unused check-prefixes

Just use default CHECK and remove duplicate RUN

llvm/test/Transforms/InstSimplify/ConstProp/overflow-ops.ll

@@ -1,8 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -instsimplify -S | FileCheck %s --check-prefixes=CHECK,CONSTPROP
-; RUN: opt < %s -instsimplify -S | FileCheck %s --check-prefixes=CHECK,INSTSIMPLIFY
-; We must *NOT* have any check-lines with prefixes other than CHECK here.
-; If we do, that means the rules are different between the passes.
+; RUN: opt < %s -instsimplify -S | FileCheck %s
 
 declare {i8, i1} @llvm.uadd.with.overflow.i8(i8, i8)
 declare {i8, i1} @llvm.usub.with.overflow.i8(i8, i8)