[mlir][arith] Delete unnecessary error logs

Function `getNeutralElement` already indicates "cannot find neutral element"
by returning nullptr through the return value, and no additional error log needs
to be output.

Author: cxy-1993

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -2544,7 +2544,6 @@ std::optional<TypedAttr> mlir::arith::getNeutralElement(Operation *op) {
           .Case([](arith::MulIOp op) { return AtomicRMWKind::muli; })
           .Default([](Operation *op) { return std::nullopt; });
   if (!maybeKind) {
-    op->emitError() << "Unknown neutral element for: " << *op;
     return std::nullopt;
   }
 

mlir/test/Dialect/Linalg/transform-tile-reduction.mlir

@@ -355,7 +355,6 @@ module {
     %0 = linalg.generic {indexing_maps = [#map, #map1], iterator_types = ["parallel", "reduction"]} ins(%arg0 : tensor<?x?xf32>) outs(%arg1 : tensor<?xf32>) {
     ^bb0(%in: f32, %out: f32):
       %1 = llvm.fmul %in, %in  : f32
-      // expected-error @below {{Unknown neutral element for:}}
       %2 = llvm.fadd %1, %out  : f32
       linalg.yield %2 : f32
     } -> tensor<?xf32>