spillHeader may be used to store offset for spill/fill instruction. It
must be infinite spill cost variable. If spillHeader gets assigned to a
register that causes fragmentation, then it could cause previously
spilled variables to not get an allocation in fail safe RA iteration.

With this change, we find first GRF candidate that can be assigned to
spillHeader. This way, we avoid fragmenting free GRF space.

(cherry picked from commit e1467856fea519d6489a20bfc65037c047b3991e)