nir/algebraic: Optimize 64bit pack/unpack
nir/algebraic: Optimize 64bit pack/unpack

This reduces the instruction count in some fp64 and int64 piglit tests

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler