[InstCombine] drop poison flags in SimplifyVectorDemandedElts

We established the (unfortunately complicated) rules for UB/poison
propagation with vector ops in:
D48893
D48987
D49047

It's clear from the affected tests that we are potentially creating 
poison where none existed before the transforms. For add/sub/mul,
the answer is simple: just drop the flags because the extra undef
vector lanes are generally more valuable for analysis and codegen.

llvm-svn: 343819