[codegen] change operand type from bool to iflag for isub borrow variants
[codegen] change operand type from bool to iflag for isub borrow variants

The type of the borrow operands for the borrow variants of the isub
instruction (isub_bin, isub_bout, isub_borrow) was bool for compatibility
reasons for isa/riscv. Since support for these instructions on RISC
architectures has been temporarily suspended, we can safely change the
type to iflags.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne