Fix consistency issue of the SIMD i/f of modcc (#278)

The `emit_gather()` function emitted the "wrong" instruction in terms of its
arguments but the instruction actually generated was correct, because
the `simd_printer` was passing the arguments to `emit_gather()` in a
different order, which was though the correct order for the finally emitted
instruction. Complicated? This commit cleans this up.

modcc/backends/avx512.hpp

@@ -121,7 +121,7 @@ struct simd_intrinsics<targetKind::avx512> {
     static void emit_gather(TextBuffer& tb, const A& addr,
                             const I& index, const S& scale) {
         tb << "_mm512_i32gather_pd(";
-        emit_operands(tb, arg_emitter(addr), arg_emitter(index),
+        emit_operands(tb, arg_emitter(index), arg_emitter(addr),
                       arg_emitter(scale));
         tb << ")";
     }

modcc/options.hpp

@@ -7,7 +7,7 @@ enum class targetKind {
     cpu,
     gpu,
     // Vectorisation targets
-    avx512,
+    avx512
  };
 
 struct Options {

modcc/simd_printer.hpp

@@ -377,7 +377,7 @@ void SimdPrinter<Arch>::visit(IndexedVariable *e) {
         value_name = emit_rawptr_name(e->index_name());
     }
 
-    simd_backend::emit_gather(text_, vindex_name, value_name, "sizeof(value_type)");
+    simd_backend::emit_gather(text_, value_name, vindex_name, "sizeof(value_type)");
 }
 
 template<targetKind Arch>