intel/fs: Fix nir_intrinsic_load_barycentric_at_sample for SIMD32.

intel/fs: Fix nir_intrinsic_load_barycentric_at_sample for SIMD32.

For uniform sample ID, only the first channel of msg_data will be
initialized.  We need to pass that component only to the SEND message
for SIMD lowering to unzip the descriptor source correctly.

Fixes several dozens of conformance test failures with SIMD32 fragment
shaders enabled, including:

dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_sample.dynamic_sample_number.*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>