Merge #631

Merge #631

631: Use builtin bool instead of creating new bool types for ComparisonExpr r=dkm a=dkm

From Mark Wielaard : https://gcc.gnu.org/pipermail/gcc-rust/2021-August/000141.html

> The TypeCheckExpr creates a new TyTy::BoolType for a
> ComparisonExpr. This new BoolType is unknown to TyTyResolveCompile
> which causes a crash when trying to compile the inferred new
> BoolType. Resolve this by looking up the builtin bool type.
> The new "bools_eq.rs" testcase uses several bools which show
> this issue.
>
> Also the lhs and rhs types need to be compatible, but don't
> need to be bool type themselves. So don't append the reference
> to the inferred type. The existing "ifunaryexpr.rs" testcase
> will fail without this fix.

Co-authored-by: Mark Wielaard <mark@klomp.org>