memops: do not mess up with phisource's source ident

In rewrite_load_instruction(), when testing if all phi-sources are
the same, the candidate is given an identifier if it hasn't one already.
But doing this inside this loop is strange:
* the pseudo may, at the end, not be selected but is changed anyway
* the identifier should be given either when the phi-source is created
  or at the end of the loop if selected.

So, do not change the identifier inside the selection loop.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>