CodeGen: correct handling of debug info generation for aliases

When aliasing a static array, the aliasee is going to be a GEP which
points to the value.  We should strip pointer casts before forming the
reference.  This was occluded by the use of opaque pointers.

This problem has existed since the introduction of the debug info
generation for aliases in b1ea0191.  The
test case would assert due to the invalid cast with or without
`-no-opaque-pointers` at that revision.

Fixes: #57179