As we implement more complex control flow, we need to emit blocks with different active lane masks, and have finer control over block generation.

Bug: b/128527271
Change-Id: Ica51bbea196b87ab442b394f0915e9a2cd375ac0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27770
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>