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

[gysit]

Summary

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.

[definelicht]

LGTM!

mlir/include/mlir/Dialect/LLVMIR/LLVMIntrinsicOps.td

 }
 def LLVM_SinOp : LLVM_UnaryIntrOpF<"sin">;
 def LLVM_RoundEvenOp : LLVM_UnaryIntrOpF<"roundeven">;
 def LLVM_RoundOp : LLVM_UnaryIntrOpF<"round">;
 def LLVM_FTruncOp : LLVM_UnaryIntrOpF<"trunc">;
 def LLVM_SqrtOp : LLVM_UnaryIntrOpF<"sqrt">;
 def LLVM_PowOp : LLVM_BinarySameArgsIntrOpF<"pow">;
 def LLVM_PowIOp : LLVM_OneResultIntrOp<"powi", [], [0,1],
-    [DeclareOpInterfaceMethods<FastmathFlagsInterface>, Pure],
-    /*requiresFastmath=*/1> {
+                                       [Pure], /*requiresFastmath=*/1> {
   let arguments =
       (ins LLVM_ScalarOrVectorOf<LLVM_AnyFloat>:$val,
            AnySignlessInteger:$power,
            DefaultValuedAttr<LLVM_FastmathFlagsAttr, "{}">:$fastmathFlags);
   let assemblyFormat = "`(` operands `)` custom<LLVMOpAttrs>(attr-dict) `:` "
       "functional-type(operands, results)";
 }
 def LLVM_BitReverseOp : LLVM_UnaryIntrOpI<"bitreverse">;