gdb, dwarf: move part of dwarf2_get_pc_bounds into separate function

gdb, dwarf: move part of dwarf2_get_pc_bounds into separate function

This commit is in preparation of the next commit.  There, we will add
a second variation to retrieve the pc bounds for DIEs tagged with
DW_TAG_entry_point.  Instead of dwarf_get_pc_bounds_ranges_or_highlow_pc
we will call a separate method for entry points.  As the validity checks
at the endo f dwarf2_get_pc_bounds are the same for both variants,
we introduced the new dwarf_get_pc_bounds_ranges_or_highlow_pc method,
outsourcing part of dwarf2_get_pc_bounds.

This commit should have no functional impact on GDB.

Approved-by: Tom Tromey <tom@tromey.com>