[MLIR] Add dialect prefix to all operations from Math

Prefix all operations from the ODS of the `Math` dialect with `Math_`
in order to avoid name clashes when including `MathOps.td` in other
TableGen files (e.g., for `FloatUnaryOp`, which also exists in
`Standard`).

Reviewed By: jpienaar, mehdi_amini

Differential Revision: https://reviews.llvm.org/D103248