add support for FrexpStruct in glsl extended instruction set.
co-authored-by: Alan Liu <firstname.lastname@example.org>
FYI, I tried to use define_inst.sh to autogen FrexpStruct in this branch but cannot make work. It seems that this script only supports spirv core instructions. Alan did this part before, I will try to sync up with him on how he makes it. But whether wonder anyone else know how to do it.