Fix float16 quantization for LSTM ops
Fix float16 quantization for LSTM ops

This CL fixes 2 issues:
1. in Quantize pass, for float16 quant, the pass replaces the original op with an identical one. This causes infinite loop for lstm ops. This CL adds checks to avoid such situations.
2. in SplitMergedOperands pass, const stateful operands are duplicated. But for float16 quantization, stateful operands have const->dequantize pattern. This CL add duplication for such pattern.
