[DAG] Use FoldConstantArithmetic for unary bitops constant folding.

BSWAP/BITREVERSE/CTPOP/CTLZ/CTLZ_ZERO_UNDEF/CTTZ/CTTZ_ZERO_UNDEF are all handled by FoldConstantArithmetic - so use directly instead of testing for isConstantIntBuildVectorOrConstantInt and relying on DAG.getNode() to perform the constant fold.