lib: Avoid the hash-lookup division if possible

lib: Avoid the hash-lookup division if possible

For Dwarf_Abbrev codes, the most common case is that they're packed at
the low end, saving uleb128 encoding size.  Since the hash table is
always resized to be no more than 90% full, those codes are always less
than the table size, and dividing for the remainder is unnecessary.

Dwarf_Dies are frequently created anew, and need to find abbrev each
time, so even that one division becomes a noticeable hotspot.  This
patch adds a branch to avoid it, which is very predictable for the CPU.

Signed-off-by: Josh Stone <jistone@redhat.com>