nir/glsl: make evaluate_rvalue() return a nir_ssa_def *

nir/glsl: make evaluate_rvalue() return a nir_ssa_def *

A long time ago, before NIR was even merged to master, glsl_to_nir used
registers and these sources were actually register sources. But nowadays
everything in glsl_to_nir is an SSA value, so stop pretending that by
evaluating an rvalue we can get an arbitrary nir_src. Most importantly,
we need this since the builder takes nir_ssa_def * sources directly.

Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>