backpack signature merging crashes when run with -ddump-rn-trace

Summary

When running a backpack signature merge, e.g. in test bkp44, with the -ddump-rn-trace flag on, the compiler crashes on loading the GREInfo

Steps to reproduce

Change test setup of bkp44 to include the -ddump-rn-trace flag

Expected behavior

What do you expect the reproducer described above to do? Not crash

Cause of the bug

The bug is cause by forcing instantiation of the GREInfo prematurely when printing traceRn from addUsedGRE that is called when processing an IEVar in lookup_ie in exports_from_avail in mergeSignatures. I have already written up a fix for it and just need someone to accept it