Fix warning raised by the compiler being too smart for its own good:

  error: loop will run at most once (loop increment never executed) [-Werror,-Wunreachable-code-loop-increment]

Bug: b/153194656
Change-Id: Idca543c13213c6e0bd2d1ff2fdbaa48348f53bf2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43989Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>