[FIRRTL] Reconsider use of inferReturnType in FIRRTLExprs

[mikeurbach]

We require FIRRTLExprs to define inferReturnType, which is used, for example, here: https://github.com/llvm/circt/blob/a3e993cb387a08e686b0df1d7f5b599bca0b3751/lib/Dialect/FIRRTL/Import/FIRParser.cpp#L2285

There is some ODS machinery to allow subclasses to customize this using inferTypeDecl and inferType: https://github.com/llvm/circt/blob/a3e993cb387a08e686b0df1d7f5b599bca0b3751/include/circt/Dialect/FIRRTL/FIRRTLExpressions.td#L548-L566

@uenoku pointed out on https://github.com/llvm/circt/pull/6701 that we can probably just use methods from InferTypeOpInterface. All of this machinery seems to predate that existing, but we might be able to simplify it now.