double-int currently operates in 128 bit integers according to double-int.h.
wide-int.h has fixed_wide_int, which does not use heap.
This helps PR126622, simplifying build_int_cst-like functions,
including the removal
of double_int_to_tree.
Beside double-int.{cc,h}, according to grep, double_int is used in:config/avr/avr.cc cp/parser.cc fixed-value.cc fixed-value.h go/go-gcc.cc emit-rtl.cc rtl.h dwarf2out.cc dwarf2out.h wide-int.cc tree-pretty-print.cc tree-pretty-print.h fold-const.cc tree.cc tree.h gengtype.cc
