radeonsi: tgsi to nir conversions (clear shaders)

What does this MR do and why?

This is the initial MR for the ongoing work to convert TGSI shaders to NIR in radeonsi. In this MR, the si_clear-* shaders have been converted to NIR. There will be a separate MR for the si_create-*-query_result_cs shaders soon.

Link to the other MRs in the series:

MR 2: !25782 (merged)

MR 3: !25972

cc: @mareko, @pepp

Edited by Ganesh Belgur Ramachandra

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