Bug 1686633 - Add a clarifying comment about commutative operations. r=yury DONTBUILD

The code in MIR.cpp takes care of reordering and there are test cases
in wasm/simd/binop-x64-ion-codegen.js, see the codegenTextX64_LITERALxv128_v128
block.  So just add a comment to avoid future confusion.

Differential Revision: https://phabricator.services.mozilla.com/D103455

js/src/jit/x86-shared/Lowering-x86-shared.cpp

@@ -796,6 +796,9 @@ void LIRGenerator::visitWasmBinarySimd128(MWasmBinarySimd128* ins) {
   MOZ_ASSERT(rhs->type() == MIRType::Simd128);
   MOZ_ASSERT(ins->type() == MIRType::Simd128);
 
+  // Note MWasmBinarySimd128::foldsTo has already specialized operations that
+  // have a constant operand, so this takes care of more general cases of
+  // reordering, see ReorderCommutative.
   if (ins->isCommutative()) {
     ReorderCommutative(&lhs, &rhs, ins);
   }