As it can be seen from accompanying cleanup, it is not unheard of
to write `~Known.Zero` meaning "what maximal value can this KnownBits
produce". But i think `~Known.Zero` isn't *that* self-explanatory,
as compared to a method with a name.
Note that not all `~Known.Zero` places were cleaned up,
only those where this arguably improves things.
This is for D60460. computeForAddSub() essentially already supports
carries because it has to deal with subtractions. This revision
extracts a lower-level computeForAddCarry() function, which allows
computing the known bits for add (carry known zero), sub (carry known
one) and addcarry (carry unknown).
As we don't seem to have any yet, I've added a unit test file for
KnownBits and exhaustive tests for the new computeForAddCarry()
functionality, as well the existing computeForAddSub() function.
Differential Revision: https://reviews.llvm.org/D60522
llvm-svn: 358297
