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[AMDGPU] Propagate dead flag during pre-RA exec mask optimizations

Authored by critson on Jul 13 2020, 8:22 PM.

Description

[AMDGPU] Propagate dead flag during pre-RA exec mask optimizations

Preserve SCC dead flags in SIOptimizeExecMaskingPreRA.
This helps with removing redundant s_andn2 instructions later.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D83637