regres: add daily run against SwiftShader/Subzero
regres: add daily run against SwiftShader/Subzero

The regres daily run will now first build and run SwiftShader/Subzero,
followed by SwiftShader/LLVM, both uploading to the same CL. This allows
us to see if there are any diffs between the two, and we can still merge
the latest LLVM version.

* Also added -dailyonly to run the daily only and quit.
* Also added -limit to limit the number of tests that are run for testing.

Bug: b/152216043
Change-Id: I5923695d39316a43f867ee0e82fb33f65a66607e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42808
Tested-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader