IR: Assert that resolve() is only called on uniqued nodes, NFC
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@226489 91177308-0d34-0410-b5e6-96231b3b80d8

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