[InstCombine] matchFunnelShift - add support for non-uniform vectors containing undefs.

Replace m_SpecificInt with m_APIntAllowUndef to matching splats containing undefs, then use ConstantExpr::mergeUndefsWith to merge the undefs together in the result.

The undef funnel shift amounts are getting replaced with zero later on - I'll address this in a later patch, otherwise we lose potential shift by splat value patterns.