intel/fs: Use shuffle_from_32bit_read at VARYING_PULL_CONSTANT_LOAD

intel/fs: Use shuffle_from_32bit_read at VARYING_PULL_CONSTANT_LOAD

shuffle_from_32bit_read can manage the shuffle/unshuffle needed
for different 8/16/32/64 bit-sizes at VARYING PULL CONSTANT LOAD.
To get the specific component the first_component parameter is used.

In the case of the previous 16-bit shuffle, the shuffle operation was
generating not needed MOVs where its results where never used. This
behaviour passed unnoticed on SIMD16 because dead_code_eliminate
pass removed the generated instructions but for SIMD8 they cound't be
removed because of being partial writes.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>