SPIR-V modules can have multiple shaders (including of the same
stage), but the global variables are all declared for the whole
module.  This can result in variables with same Binding but
incompatible types, so those need to be removed before we use.

Previously, a similar issue but with a narrower scope was fixed by
6775665e ("spirv: Eliminate dead input/output variables after
translation.").

This patch depends on the previous patch that prevents variables used
only in pointer initializers to be considered dead.

Closes: mesa/mesa#3686
Fixes: 3a266a18

 ("nir/spirv: Add support for declaring variables")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <mesa/mesa!8133>