[InstCombine] Extend (add (sext x), cst) --> (sext (add x, cst')) and (add (zext x), cst) --> (zext (add x, cst')) to work for vectors

Differential Revision: https://reviews.llvm.org/D51236

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340796 91177308-0d34-0410-b5e6-96231b3b80d8