[AMDGPU] Collapse adjacent SI_END_CF

Add a pass to remove redundant S_OR_B64 instructions enabling lanes in
the exec. If two SI_END_CF (lowered as S_OR_B64) come together without any
vector instructions between them we can only keep outer SI_END_CF, given
that CFG is structured and exec bits of the outer end statement are always
not less than exec bit of the inner one.

This needs to be done before the RA to eliminate saved exec bits registers
but after register coalescer to have no vector registers copies in between
of different end cf statements.

Differential Revision: https://reviews.llvm.org/D35967

llvm-svn: 309762