[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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343819
91177308-0d34-0410-b5e6-
96231b3b80d8
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