IR: Assert that resolve() is only called on uniqued nodes, NFC

Add an assertion in `UniquableMDNode::resolve()` to prevent temporaries
from being resolved (once they're merged back in).  Needed to shuffle
order of `resolve()` and `storeDistinctInContext()` to prevent it from
firing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226489 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed