Don't allocate unnecessarily in APInt::operator[+-]. NFC.

APInt::operator+(uint64_t) just forwarded to operator+(const APInt&).

Constructing the APInt for the RHS takes an allocation which isn't
required.  Also, for APInt's in the slow path, operator+ would
call add() internally which iterates over both arrays of values.  Instead
we can use add_1 and sub_1 which only iterate while there is something to do.

Using the memory for 'opt -O2 verify-uselistorder.lto.opt.bc -o opt.bc'
(see r236629 for details), this reduces the number of allocations from
23.9M to 22.7M.

llvm-svn: 270959