[X86] Cleanup ISD::BRIND handling code in X86DAGToDAGISel::Select. NFC

-Drop llvm:: on MVT::i32
-Use getValueType instead of getSimpleValueType for an equality
check just cause its shorter and doesn't matter.
-Don't create a const SDValue & since its cheap to copy.
-Remove explicit case from MVT enum to EVT.
-Add message to assert.