[InstCombine] canNarrowShiftAmt - replace custom Constant matching with m_SpecificInt_ICMP

The existing code ignores undef values which matches m_SpecificInt_ICMP, although m_SpecificInt_ICMP returns false for an all-undef constant, I've added test coverage at rGfe0197e194a64f9 to show that undef folding should already have dealt with that case.