[X86] If the carry input to an addcarry/subborrow intrinsic is known to be 0,...

[X86] If the carry input to an addcarry/subborrow intrinsic is known to be 0, emit a flag setting ADD/SUB instead of ADC/SBB.

Previously we had to take the carry in and add -1 to it to set the carry flag so we could use it with ADC/SBB. But if we know its 0 then we don't need to bother.

This should go a long way towards fixing PR24545.

llvm-svn: 348727