[X86] X86::isConstantSplat - enable partial undef bit handling by default.

We currently only ever use this for lowering constant uniform values (shift/rotate by immediate) so we can safely enable it by default (it treats the undef bits as zero when extracting constants).

This is necessary for an upcoming patch that will use SimplifyDemandedBits more aggressively on funnel shift amounts and causes regressions in vXi64 constant without it.