(NilCheck ...) => (LoweredNilCheck ...)
(IsNonNil ptr) => (NotEqual (CMPconst [0] ptr))
(IsInBounds      idx len) => (LessThanU  (CMP idx len))
(IsSliceInBounds idx len) => (LessEqualU (CMP idx len))

// pseudo-ops
(GetClosurePtr ...) => (LoweredGetClosurePtr ...)
(GetCallerSP   ...) => (LoweredGetCallerSP   ...)
(GetCallerPC   ...) => (LoweredGetCallerPC   ...)

// Absorb pseudo-ops into blocks.