Previously each constant would cause new memory to be allocated. This
change makes us search through the existing constants to check if a
suitable one already exists.

Bug: b/178661423
Change-Id: I1315c80f019ff51e96245238100aa672d5e74930
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52229
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>