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nir: Make nir_deref_path_init skip trivial casts

Merged Caio Oliveira requested to merge cmarcelo/mesa:r/nir-deref-path-skip-trivial-casts into master

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 !3047 (comment 383660).

Edited by Caio Oliveira