[TableGen] Hoist the code for copying InstRWs from an old scheduling class to a new...

[TableGen] Hoist the code for copying InstRWs from an old scheduling class to a new one out of the loop that assigns instructions to the new class. NFCI

We already know all the of instructions we're processing in the instruction loop belong to no class or all to the same class. So we only have to worry about remapping one class. So hoist it all out and remove the SmallPtrSet that tracked which class we'd already remapped.

I had to introduce new instruction loop inside this code to print an error message, but that only occurs on the error path.

llvm-svn: 328142