Current implementation of skip insertion (SIInsertSkip) makes it a mandatory pass
required for correctness. The idea was to have this handling as 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. SIRemoveShortExecBranches tries to
remove the skips for short branches. It also tries to retain s_cbranch_execz
for the cases where the skip branch may be necessary.
The new pass, SIRemoveShortExecBranches will replace the handling of
skip insertion in the existing SIInsertSkip Pass.