[MLIR] Split arith dialect from the std dialect

[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