[InstCombine] Extend (add (sext x), cst) --> (sext (add x, cst')) and (add...

[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

llvm-svn: 340796