i965/gen6: Force WHILE exec size to 8.

i965/gen6: Force WHILE exec size to 8.

We can't just look at the instruction that happens to appear at the
start of the loop, because it might be some other exec size and cause
us to only loop on the first N channels.  We always want 8 in our
current code (since 16 doesn't work so we don't do 16-wide fragment in
that case).

Fixes loop-03.vert, which was triggering the assertions.