[AMDGPU] Invert the handling of skip insertion.

The current implementation of skip insertion (SIInsertSkip) makes it a
mandatory pass required for correctness. Initially, the idea was to
have an optional pass. This patch inserts the s_cbranch_execz upfront
during SILowerControlFlow to skip over the sections of code when no
lanes are active. Later, SIRemoveShortExecBranches removes the skips
for short branches, unless there is a sideeffect and the skip branch is
really necessary.

This new pass will replace the handling of skip insertion in the
existing SIInsertSkip Pass.

Differential revision: https://reviews.llvm.org/D68092

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