nir: Make nir_deref_path_init skip trivial casts

In a NIR generated using SPIR-V initializers to variables, copy
propagation can end up transforming

    vec1 32 ssa_33 = deref_var &@1 (shared mat2x4)
    vec1 32 ssa_35 = mov ssa_33
    vec1 32 ssa_7 = deref_cast (mat2x4 *)ssa_35 (shared mat2x4)  /* ptr_stride=0 */

into

    vec1 32 ssa_33 = deref_var &@1 (shared mat2x4)
    vec1 32 ssa_7 = deref_cast (mat2x4 *)ssa_33 (shared mat2x4)  /* ptr_stride=0 */

Before the optimization, the "head" of a path of deref that uses ssa_7
will be the cast.  After, it will be the variable in ssa_33.  Since
the types are the same, this is a trivial cast that would be picked up
by nir_opt_deref.

If we need to compare such deref-chain after optimization with another
deref-chain for the same variable, the compare function will get
confused by the cast in the middle.

One alternative would be to add nir_opt_deref to places that compare
derefs, but that might not scale well, so skip the trivial casts when
generating the paths instead.

Motivated by the discussion in
https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3047#note_383660.

Reviewed-by: Jason Ekstrand <[email protected]>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3420>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3420>

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