symbol_table: Prehash the key on insert, and reuse the entry on shadowing.

symbol_table: Prehash the key on insert, and reuse the entry on shadowing.

Mostly saves computing the hash twice, but while we're here there's no
need for shadowing to walk the table again.

Release Mesa build runtime of
KHR-Single-GL46.arrays_of_arrays_gl.SizedDeclarationsPrimitive -4.19869%
+/- 3.20231%

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22451>