Add op_values() to iterate over the SDValue operands of an SDNode.

Add op_values() to iterate over the SDValue operands of an SDNode.

SDNode already had ops() which would iterate over the operands and return
SDUse*.  This version instead gets the SDValue's out of the SDUse's so that
we can use foreach in more places.

Reviewed by David Blaikie.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240805 91177308-0d34-0410-b5e6-96231b3b80d8