i965/fs: Use WE_all for gl_SampleID header register munging.

i965/fs: Use WE_all for gl_SampleID header register munging.

This code should execute without regard to the currently executing
channels.  Asking for gl_SampleID inside control flow might break in
strange ways.  It appears to break even at the top of the program in
SIMD16 mode occasionally as well.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: mesa-stable@lists.freedesktop.org