[ConstantFolding] Fixing addo/subo with undef

When folding addo/subo with undef, the current
convention is to use { -1, false } for addo and
{ 0, false } for subo. This was fixed for InstSimplify in
https://reviews.llvm.org/rGf094d65beaa492e845b03561eddd75b5be653a01,
but not in ConstantFolding.

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D99564