Disable the InstructionCombiningPass and O2 optimization level to make
ReactorUnitTests.LargeStack take only a few seconds to run. Otherwise,
it would take 6 minutes on my desktop workstation.

Bug: b/174031014
Change-Id: I88fb240fe1ba6dcf822a8ff70bb595a4c3f3aa01
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50710
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>