Keying off a single size_t is likely to cause collisions, more so when the hash function is simple shift-xors of 3 32-bit integers.

Use a proper cache key comparision and a better hash function while we're at it.

Bug: b/137649247
Change-Id: I1e0deefb0976102714d43dcb9dcedf6aa809bf7f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34909Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
(cherry picked from commit f046402b)
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34928