Demote this opt remark to DEBUG.

From a random opt-stat output:

Top 10 remarks:
  tailcallelim/tailcall          53%
  inline/AlwaysInline            13%
  gvn/LoadClobbered              13%
  inline/Inlined                  8%
  inline/TooCostly                2%
  inline/NoDefinition             2%
  licm/LoadWithLoopInvariantAddressInvalidated  2%
  licm/Hoisted                    1%
  asm-printer/InstructionCount    1%
  prologepilog/StackSize          1%

llvm-svn: 319235