x86: improve resolution of register equates

x86: improve resolution of register equates

Allow transitive (or recursive) equates to work in addition to direct
ones. The only requirements are that
- the equate being straight of a register, i.e. no expressions involved
  (albeit I'm afraid something like "%eax + 0" will be viewed as %eax),
- at the point of use there's no forward ref left which cannot be
  resolved, yet.