nir: Fix load_const comparisons for CSE.

nir: Fix load_const comparisons for CSE.

We want the size of a float per component, not the size of a whole vec4.

NIR instructions on i965:
total instructions in shared programs: 1261937 -> 1261929 (-0.00%)
instructions in affected programs:     114 -> 106 (-7.02%)

Looking at one of these examples (tesseract), it's from vec4 load_consts
for a MRT solid fill, which do get CSEed now that we don't memcmp off the
end of the const value and into the SSA def.  For the 1-component loads
that are common in i965, we were only memcmping off into the rest of the
usually zero-filled const_value.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>