[gdb/tdep] Factor out amd64_get_used_input_int_regs

The function amd64_get_unused_input_int_reg consists of two parts:
- finding the used int registers in an insn, and
- picking an unused int register.

Factor out the first part as new function amd64_get_used_input_int_regs.

No functional changes.

Tested on x86_64-linux.