cmd/gc: Make sure bools lose idealness when used with logical operators.

Bools from comparisons can be assigned to all bool types, but this idealness would propagate through logical operators when the result should have been lowered to a non-ideal form.

Fixes #3924.

R=golang-dev, remyoudompheng, r, rsc, mtj
CC=golang-dev
https://golang.org/cl/6855061
2 files changed