i965/vec4: Fix SSBO loads for 64-bit data
i965/vec4: Fix SSBO loads for 64-bit data

Same requirements as for UBO loads.

v2:
  - use byte_offset() instead of offset() (Iago)
  - keep the const. offset as an immediate like the original code did (Juan)

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler