intel/compiler: Relax mixed type restriction for saturating immediates

At the time of commit 7bc6e455 (i965: Add support for saturating
immediates.) we thought mixed type saturates would be impossible.  We
were only thinking about type converting moves from D to F, for
example.  However, type converting moves w/saturate from F to DF are
definitely possible.  This change minimally relaxes the restriction to
allow cases that I have been able trigger via piglit tests.

Fixes new piglit tests:
 - arb_gpu_shader_fp64/execution/built-in-functions/fs-sign-sat-neg-abs.shader_test
 - arb_gpu_shader_fp64/execution/built-in-functions/vs-sign-sat-neg-abs.shader_test

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit f8e54d02)