i965: Set execution size to 8 for instructions with force_sechalf set.

i965: Set execution size to 8 for instructions with force_sechalf set.

Both inst->force_uncompressed and inst->force_sechalf mean that the
generated instruction should be uncompressed and have an execution size
of 8.  We don't require the visitor to set both flags - setting
inst->force_sechalf by itself is supposed to be enough.

On Gen4-7, guess_execution_size() demoted instructions to 8-wide based
on the default compression state.  On Gen8+, we instead set a default
execution size, which worked great...except that we forgot to check
inst->force_sechalf when deciding whether to use 8 or 16.

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