[Moore] Fix mem2reg for variables with non-packed types

Only attempt to promote variables with packed types during mem2reg. Also
run the `basic.sv` test of ImportVerilog through `circt-verilog` as a
sanity check of the transformations done by the tool.

include/circt/Dialect/Moore/MooreOps.td

@@ -462,7 +462,7 @@ def NamedConstantOp : MooreOp<"named_constant", [
   }];
 }
 
-def StringConstantOp : MooreOp<"string_constant", [Pure, ConstantLike]> {
+def StringConstantOp : MooreOp<"string_constant", [Pure]> {
   let summary = "Produce a constant string value";
   let description = [{
     Produces a constant value of string type.

lib/Dialect/Moore/MooreOps.cpp

@@ -322,6 +322,12 @@ SmallVector<MemorySlot> VariableOp::getPromotableSlots() {
   if (mlir::mayBeGraphRegion(*getOperation()->getParentRegion()) ||
       getInitial())
     return {};
+
+  // Ensure that `getDefaultValue` can conjure up a default value for the
+  // variable's type.
+  if (!isa<PackedType>(getType().getNestedType()))
+    return {};
+
   return {MemorySlot{getResult(), getType().getNestedType()}};
 }
 

test/Conversion/ImportVerilog/basic.sv

@@ -1,4 +1,5 @@
 // RUN: circt-translate --import-verilog %s | FileCheck %s
+// RUN: circt-verilog --ir-moore %s
 // REQUIRES: slang
 
 // Internal issue in Slang v3 about jump depending on uninitialised value.