We now have safepoints at nearly all the instructions. When
GOEXPERIMENT=clobberdead is on, it inserts clobbers nearly at
every instruction. Currently this doesn't work. (Maybe the stack
maps at non-call safepoints are still imprecise. I haven't
investigated.) For now, only use call-based safepoints if the
experiment is on.

Updates #27326.

Change-Id: I72cda9b422d9637cc5738e681502035af7a5c02d
Reviewed-on: https://go-review.googlesource.com/131956Reviewed-by: Keith Randall <khr@golang.org>