i965: Fix math instruction hstride assertions on Broadwell.

i965: Fix math instruction hstride assertions on Broadwell.

In the final revision of my gen8_generator patch, I updated the MATH
instruction's assertion from (dst.hstride == 1) to check that source and
destination hstride matched.  Unfortunately, I didn't test this enough,
and many Piglit tests fail this test.

The documentation indicates that "scalar source is also supported",
which we believe means <0,1,0> access mode (hstride == 0).  If hstride
is non-zero, then it must match the destination register.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>