GCC expands switch statements in a very simplistic way and tries to use a table

GCC expands switch statements in a very simplistic way and tries to use a table
expansion even when it is a bad idea for performance or codesize.
GCC typically emits extremely sparse tables that contain mostly default entries
(something which currently cannot be tuned by backends).  Additionally the
computation of the minimum/maximum label offsets is too simplistic so the
tables are often twice as large as necessary.

The cost of a table switch is significant due to the setup overhead, the table
lookup (which due to being sparse and large adds unnecessary cachemisses)
and hard to predict indirect jump.  Therefore it is best to avoid using a table
unless there are many real case labels.

This patch fixes that by setting the default aarch64_case_values_threshold to
16 when the per-CPU tuning is not set.  On SPEC2006 this improves the switch
heavy benchmarks GCC and perlbench both in performance (1-2%) as well as size
(0.5-1% smaller).

    gcc/
	* config/aarch64/aarch64.c (aarch64_case_values_threshold):
	Return a better case_values_threshold when optimizing.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@236771 138bc75d-0d04-0410-961f-82ee72b054a4