TargetLowering: fix an infinite DAG combine in SimplifySETCC

TargetLowering::SimplifySetCC wants to swap the operands of a SETCC to
canonicalize the constant to the RHS. The bug here was that it did so whether
or not the RHS was already a constant, leading to an infinite loop.

rdar://111847838

Differential revision: https://reviews.llvm.org/D155095