cmd/compile: update comment about x86 nop instruction generator

The comment about losing the high bits is incorrect.  We now use these
nops in places where they really need to be a nop.  (Before inline
marks, we used them just before deferreturn calls, so they could
clobber any caller-saved values.)

Change-Id: I433d1ec455aa37dab8fef6eb7d407f3737dbb97f
Reviewed-on: https://go-review.googlesource.com/c/158057


Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>