See ISA, 4.7.1.3 for details.

Window check is inserted before commands that push "used register
watermark" beyond its current level. Used register watermark is reset on
instructions that change WINDOW_BASE/WINDOW_START SRs.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>