[DAGCombine] Limit 'hoist add/sub binop w/ constant op' to non-opaque consts

I don't have a test case for these, but there is a test case for D62266
where, even after all the constant-folding patches, we still end up
with endless combine loop. Which makes sense, since we don't constant
fold for opaque constants.

llvm-svn: 362156