They have to read the boolean into a register first and then do
the comparison.

Fixes #10598.

Change-Id: I2b808837a8c6393e1e0778296b6592aaab2b04bf
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9453Reviewed-by: Dave Cheney <dave@cheney.net>