i965/fs: Fix the inline nir_op_pack_double optimization
i965/fs: Fix the inline nir_op_pack_double optimization

We can only do the optimization if the source *is* SSA.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Cc: "13.0 17.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler