Use IO for uniqueSupply in SimplM

When already operating in an IO Monad it's generally "better" (faster) to conjure up new Uniques from the RealWorld.

The idea is described in #16843 (closed) and implemented for some monads in !1502 (closed)

Since SimplM already has a MonadIO instance it seems like a safe bet that we should rather use that for generating uniques. Rather than carry an supply around.

I will write up a patch.

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