nir/tex_instr_create: Initialize all 4 sources
nir/tex_instr_create: Initialize all 4 sources

This helps a lot with things like lowering passes that may need to add
sources.

Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler