This change adds a callback mechanism to report how many instructions
of certain types are left after the optimization passes have been run.
This enables unit tests to check that the desired optimization actually
took place, instead of just checking correct execution results.

Bug: b/180665600
Change-Id: I3916d327138516a0a0778be2b3fdd5b000fc9bdb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52989
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>