Implement DW_OP_{bregx,consts,const8s,const8u,constu} in dwarfexpr_to_dag
readdwarf.c (dwarfexpr_to_dag) didn't hanle various DW_OP expressions
causing Warning: DWARF2 CFI reader: unhandled DW_OP_ opcode and errors
m_debuginfo/readdwarf.c:2822 (copy_convert_CfiExpr_tree): Assertion
'srcix >= 0 && srcix < VG_(sizeXA)(srcxa)' failed.
Implement DW_OP_bregx and DW_OP_consts as reported in bug #461074.
Also add implementations for DW_OP_const8s, DW_OP_const8u and
DW_OP constu.
https://bugs.kde.org/show_bug.cgi?id=461074