i965/vec4: Add unit tests for cmod propagation pass

i965/vec4: Add unit tests for cmod propagation pass

This include the same tests coming from test_fs_cmod_propagation, (non
vector glsl types included) plus some new with vec4 types, inspired on
the regressions found while the optimization was a work in progress.

Additionally, the check of number of instructions after the
optimization was changed from EXPECT_EQ to ASSERT_EQ. This was done to
avoid a crash on failing tests that expected no optimization, as after
checking the number of instructions, there were some checks related to
this last instruction opcode/conditional mod.

v2: update tests after Matt Turner's review of the optimization pass
v3: tweaks on the tests (mostly on the comments), after Matt Turner's
    review

Reviewed-by: Matt Turner <mattst88@gmail.com>