R600/SI: Initialize unused VOP3 sources to 0 instead of SIOperand.ZERO

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213563 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Initailize encoding fields of unused VOP3 modifiers to 0

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213564 91177308-0d34-0410-b5e6-96231b3b80d8