nir/spirv: Handle derefs in vtn_ssa_value
nir/spirv: Handle derefs in vtn_ssa_value

This is kind of a hack, but it makes vtn_ssa_value insert a load if the
value requested is actually a deref.  This shouldn't happen normally but,
thanks to the impedence mismatch of the NIR function parameter model vs.
the SPIR-V model, this can happen for function arguments.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler