do not emit unnecessary "| 0" for bitops
regen bootstrap compiler (so that the prev. commit takes effect)
clarify the reasoning behind the returned types of binary ops
add a failing test-case (const-int % 0 should be folded to NaN)
[bugfix] int % 0 should be const-folded to NaN (instead of to zero), as clarified by a test-case in the prev commit