This change finalizes the series of patches aiming to replace the old strategy of VGPR to SGPR copy lowering.
- Following the https://reviews.llvm.org/D128252 and https://reviews.llvm.org/D130367 code parts that are no longer used were removed.
- The first pass over the MachineFunctoin collects all the necessary information.
- Lowering is done in 3 phases:
- VGPR to SGPR copies analysis lowering
- REG_SEQUENCE, PHIs, and SGPR to VGPR copies lowering
- SCC copies lowering is done in a separate pass over the Machine Function
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