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 <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler