[ConstantFolding] Fix 'undef' folding for @llvm.[us]{add,sub}.with.overflow ops (PR43188)
As we have already established/fixed in
https://bugs.llvm.org/show_bug.cgi?id=42209
https://reviews.llvm.org/D63065
https://reviews.llvm.org/rL363522
the InstSimplify handling for @llvm.with.overflow ops with undefs
is correct. Therefore if ConstantFolding produces different results,
then it is wrong.
This duplication of code hints at the need for some refactoring,
but for now address the brokenness of ConstantFolding by
copying the known-good handling from rL363522.
Fixes https://bugs.llvm.org/show_bug.cgi?id=43188
llvm-svn: 370608