This is a preparatory step for D34515
- makes nodes ISD::ADDCARRY and ISD::SUBCARRY legal for i32
- lowering is done by first converting the boolean value into the carry flag using (_, C) ← (ARMISD::ADDC R, -1) and converted back to an integer value using (R, _) ← (ARMISD::ADDE 0, 0, C). An ARMISD::ADDE between the two operations does the actual addition.
- for subtraction, given that ISD::SUBCARRY second result is actually a borrow, we need to invert the value of the second operand and result before and after using ARMISD::SUBE. We need to invert the carry result of ARMISD::SUBE to preserve the semantics.
- given that the generic combiner may lower ISD::ADDCARRY and ISD::SUBCARRYinto ISD::UADDO and ISD::USUBO we need to update their lowering as well otherwise i64 operations now would require branches. This implies updating the corresponding test for unsigned.
- add new combiner to remove the redundant conversions from/to carry flags to/from boolean values (ARMISD::ADDC (ARMISD::ADDE 0, 0, C), -1) → C