iris: Implement ARB_compute_variable_group_size (push constant way)

Some compiler and the i965 support already landed on master, this will track Iris support for the extension.

The only missing thing here is the suggestion in the original MR to compile all the SIMD variants for the variable group size case -- I'll get to that either as an addition to this MR or another MR, depending on the feedback.

Edited by Caio Oliveira

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