R600: Add a pass that merge Vector Register

Previously commited @183279 but tests were failing, reverted @183286
It was broken because @183336 was missing, now it's there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183343 91177308-0d34-0410-b5e6-96231b3b80d8