Added bitfield code-gen bug
Warn on struct/union-with-flex-array embedded in s/u/array
v_freeup_reg() maintains the vstack's type_ref

This fixes a problem shown by compound assignment to a bitfield -
register usage and caching caused the v_freeup_reg() call to ditch a
vstack's type when replacing.
Disallow NULL types for vpush() and v_clear()
Merge branch 'fix/bitfield-codegen-or'