i965: Don't try to decode types for non-existent src1.

i965: Don't try to decode types for non-existent src1.

KHR-GL45.shader_ballot_tests.ShaderBallotBitmasks has a MOV that hits
this validation path.  MOVs don't have a src1 file, but calling
brw_inst_src1_type() was tripping on src1.file being BRW_IMMEDIATE_VALUE
and the hw_type being something invalid for immediates.

To work around this, just pretend src1 is src0 if there isn't a src1.

Fixes: 2572c2771d0cab0b9bc489d354ede44dfc88547b (i965: Validate "Special
       Requirements for Handling Double Precision Data Types")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102680
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>