[mlir][llvm] Add FastmathFlagsInterface only once (NFC).

Previously, the FastmathFlagsInterface has been added twice to
the LLVM::PowIOp. Once directly and once indirectly using the
requiresFastmath flag. This revision only uses the flag and drops
the redundant interface.

Reviewed By: definelicht

Differential Revision: https://reviews.llvm.org/D144702