[DebugInfo] Pre-commit of test case for DW_OP_breg/DW_OP_fbreg folds

This currently triggers undefined behavior if executed with an
ubsan build. It is just a precommit of the test case to show that
we got a problem.

Fix is proposed in https://reviews.llvm.org/D67263 and plan is to
commit the fix directly after this patch.

llvm-svn: 371303