[InstCombine] Sanitize undef vector constant to 1 in X*(2^C) with X << C (PR47133)
While x*undef is undef, shift-by-undef is poison,
which we must avoid introducing.
Also log2(iN undef) is *NOT* iN undef, because log2(iN undef) u< N.
lebedev.ri on Aug 12 2020, 11:06 AM.Authored by