[MLIR] Split arith dialect from the std dialect

Create the Arithmetic dialect that contains basic integer and floating

point arithmetic operations. Ops that did not meet this criterion were

moved to the Math dialect.

First of two atomic patches to remove integer and floating point

operations from the standard dialect. Ops will be removed from the

standard dialect in a subsequent patch.

Reviewed By: ftynse, silvas

Differential Revision: https://reviews.llvm.org/D110200