Add new whole-instr register iterator

[edeiana]

Currently we don't have a way to iterate over the register operands of an instr_t directly.
To do so, we have to:

1. obtain the number of src (instr_num_srcs()) and dst (instr_num_dsts()) operands;
2. iterate over and obtain the operand (with instr_get_src() and instr_get_dst());
3. obtain the number of registers of every operand with opnd_num_regs_used();
4. iterate over and obtain the reg_id_t register with opnd_get_reg_used().

We find this operation common enough to warrant its own APIs in core/ir/instr_shared.c to avoid code duplication.

Since we still want to distinguish between source register operands and destination register operands of an instr_t, we'd want the following 4 APIs:

uint instr_num_src_reg(instr_t *instr);
uint instr_num_dst_reg(instr_t *instr);
reg_id_t instr_get_src_reg(instr_t *instr, uint index);
reg_id_t instr_get_dst_reg(instr_t *instr, uint index);

Note that registers in destination operands that are memory references are still being read, hence they should be counted among the source register operands of an instruction.