Instead of casting to int for a map key.

Bug: b/147359661
Change-Id: Id777f7285f9349e7ab855c4a5a26d6a2aa4c02ba
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39953
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>