i965/fs: fix copy/constant propagation regioning checks

i965/fs: fix copy/constant propagation regioning checks

We were not accounting for subreg_offset in the check for the start
of the region.

Also, fs_reg::regs_read() already takes the stride into account, so we
should not multiply its result by the stride again. This was making
copy-propagation fail to copy-propagate cases that would otherwise be
safe to copy-propagate. Again, this was observed in fp64 code, since
there we use stride > 1 often.

v2 (Sam):
- Rename function and add comment (Jason, Curro).
- Assert that register files and number are the same (Jason).
- Fix code to take into account the assumption that src.subreg_offset
is strictly less than the reg_offset unit (Curro).
- Don't pass the registers by value to the function, use
'const fs_reg &' instead (Curro).
- Remove obsolete comment in the commit log (Curro).

v3 (Sam):
- Remove the assert and put the condition in the return (Curro).
- Fix function name (Curro).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>