Add new whole-instr register iterator

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.