Simplify expression pass

[hanchenye]

Based on my experience, MLIR does not have passes to simplify things like this:

%cst_1 = 1.0 : f32
%1 = mulf %0, %cst_1 : f32

In this case, all uses of %1 can be replaced by %0. This simplification pass is meaningful for us because this is automatically simplified in Vivado HLS, which makes our estimator wrongly take some redundant mulf or other operations into consideration.