cmd/compile: fix typechecking logical operators panic with non-boolean operand

In CL 255899, we added code to make clearer error when non-bool used
as operand to logical operators. The code is safe, because node type
is guaranteed to be non-nil.

In CL 279442, we refactored typechecking arith, including moving
typechecking logical operators to separate case. Now we have to
explicitly check if operand type is not nil, because calling Expr can
set operand type nil for non-bool operands.

Fixes #45804

Change-Id: Ie2b6e18f65c0614a803b343f60e78ee1d660bbeb
Reviewed-on: https://go-review.googlesource.com/c/go/+/314209


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>