Don't passively concatenate MDNodes

I have audited all the callers of concatenate and none require duplicate
entries to service concatenation.
These duplicates serve no purpose but to needlessly embiggen the IR.

N.B. Layering getMostGenericAliasScope on top of concatenate makes it
O(nlogn + mlogm) instead of O(n*m).

llvm-svn: 278836