[SV] Mark sv.xmr.ref op as pure (#6260)

Having an `sv.xmr.ref` op inside a procedural block such as
`sv.alwayscomb` triggers an assertion in `PrepareForEmission`. The pass
would identify the ref op as having side-effects and then go ahead and
try to pull it outside the procedural block. Doing so would create a
`sv.reg` op with multiple nested inout types, which breaks.

Fix the issue by marking the `sv.xmr.ref` op as pure. Taking a reference
to something does not have a side-effect. It's accessing what's behind
the reference that has side-effects.

include/circt/Dialect/SV/SVInOutOps.td

@@ -110,7 +110,10 @@ def XMROp : SVOp<"xmr", []> {
   let assemblyFormat = "(`isRooted` $isRooted^)? custom<XMRPath>($path, $terminal) attr-dict `:` qualified(type($result))";
 }
 
-def XMRRefOp : SVOp<"xmr.ref", [DeclareOpInterfaceMethods<SymbolUserOpInterface>]> {
+def XMRRefOp : SVOp<"xmr.ref", [
+  DeclareOpInterfaceMethods<SymbolUserOpInterface>,
+  Pure
+]> {
   let summary = "Encode a reference to something with a hw.hierpath.";
   let description = [{
     This represents a hierarchical path, but using something which the compiler

test/Conversion/ExportVerilog/prepare-for-emission.mlir

@@ -272,3 +272,23 @@ hw.module @Issue5605(in %a: i1, in %b: i1, in %clock: i1, in %reset: i1) {
   sv.assert.concurrent posedge %clock, %reset label "assert_1" message "bar"(%1) : i2
   hw.output
 }
+
+// -----
+
+// The following use of `sv.xmr.ref` inside a procedural block would trigger an
+// assertion in `PrepareForEmission`, since the op was not marked as side-effect
+// free.
+//
+// CHECK-LABEL: hw.module @Foo
+module attributes {circt.loweringOptions = "disallowLocalVariables"} {
+  hw.module @Foo(in %a: i1) {
+    hw.wire %a sym @a : i1
+    // CHECK: sv.alwayscomb
+    sv.alwayscomb {
+      // CHECK-NEXT: sv.xmr.ref
+      %0 = sv.xmr.ref @xmr : !hw.inout<i1>
+      sv.verbatim "{{0}}" (%0) : !hw.inout<i1>
+    }
+  }
+  hw.hierpath @xmr [@Foo::@a]
+}