[InstSimplify/NewGVN] Add option to control the use of undef.

Making use of undef is not safe if the simplification result is not used
to replace all uses of the result. This leads to problems in NewGVN,
which does not replace all uses in the IR directly. See PR33165 for more
details.

This patch adds an option to SimplifyQuery to disable the use of undef.

Note that I've only guarded uses if isa<UndefValue>/m_Undef where
SimplifyQuery is currently available. If we agree on the general
direction, I'll update the remaining uses.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D84792