[IR][GVN] allow intrinsics in Instruction's isCommutative query

As discussed in D86798 / rG09652721 , we were potentially
returning a different result for whether an Instruction
is commutable depending on if we call the base class or
derived class method.

This requires relaxing an assert in GVN, but that pass
seems to be working otherwise.

NewGVN requires more work because it uses different
code paths for numbering binops and calls.