[AMDGPU] Always select s_cselect_b32 for uniform 'select' SDNode

This patch contains changes necessary to carry physical condition register (SCC) dependencies through the SDNode scheduler.  It adds the edge in the SDNodeScheduler dependency graph instead of inserting the SCC copy between each definition and use. This approach lets the scheduler place instructions in an optimal way placing the copy only when the dependency cannot be resolved.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D133593