Move ExtractVectorElements to SelectionDAG.

This seems generally useful, and makes sense to
go along with SplitVector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206041 91177308-0d34-0410-b5e6-96231b3b80d8
Debug info: Store the DIVariable in DebugLocEntry also for constants,
so DwarfDebug::emitDebugLocEntry can emit them with the correct signedness.

rdar://problem/15928306

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206042 91177308-0d34-0410-b5e6-96231b3b80d8