util/hash_table: added hash functions for integer types

A few hash_table users roll their own integer hash functions which
call _mesa_hash_data to perform the hashing which ultimately calls
into XXH32 with a dynamic key length. When using small keys with a
constant size the hash rate can be greatly improved by inlining
XXH32 and providing it a constant key length, see:
https://fastcompression.blogspot.com/2018/03/xxhash-for-small-keys-impressive-power.html

Additionally, this patch removes calls to _mesa_key_hash_string and
makes them instead call _mesa_has_string directly, matching the new
integer hash functions.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3475>

src/compiler/glsl/lower_output_reads.cpp

@@ -72,7 +72,7 @@ static unsigned
 hash_table_var_hash(const void *key)
 {
    const ir_variable * var = static_cast<const ir_variable *>(key);
-   return _mesa_key_hash_string(var->name);
+   return _mesa_hash_string(var->name);
 }
 
 output_read_remover::output_read_remover(unsigned stage)