compiler: avoid generating unnamed bool type descriptor

We were generating it in cases where a boolean expression was
converted directly to an empty interface type.

Fixes golang/go#40152

Change-Id: I59e0f184b46fa1c1309b322b4b14e93f3fb0436f
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/242002
Reviewed-by: Cherry Zhang <cherryyz@google.com>