Recently a new optimization was added to the DAGCombiner which exposed a problem with SDNode memoization (see bug report at https://bugs.llvm.org/show_bug.cgi?id=47092). The problem was that an existing ISD::ADD node node with NSW/NUW flags set was reused when the DAGCombiner produced an identical node except without any guarantee for no overflow. This led to false NSW/NUW flags on the resulting SystemZ MachineInstr, and since SystemZElimCompare.cpp trusts these flags when eliminating a compare with 0, wrong code resulted.
This patch changes the default behavior in these cases to clear any NSW/NUW flags in case they are not set by the caller of getNode(). This seems safer since otherwise wrong-code might result any time an optimization forgets to explicitly clear these flags.
This doesn't cause any test failures, and changes just one locr instruction on SPEC (cse.s) from locrlh to locrne (in that case an SRK instruction had an NSW flag which it seems it perhaps should not have...)
I am not sure about the FP flags so I left them out of this change, but maybe one or more of them also should be conservatively cleared by default?