[AMDGPU] Add SI_EARLY_TERMINATE_SCC0 for early terminating shader

Add pseudo instruction to allow early termination of pixel shader
anywhere based on the value of SCC.  The intention is to use this
when a mask of live lanes is updated, e.g. live lanes in WQM pass.
This facilitates early termination of shaders even when EXEC is
incomplete, e.g. in non-uniform control flow.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D88777