Compiler: fix remaining .parameters[1] sites for ConstType
Fix three additional call sites in utilities.jl (singleton_type,
maybe_singleton_const) and inferenceresult.jl that accessed
.parameters[1] after isconstType checks without using consttype_param.
Also use typeof(t) === Core.ConstType instead of isa(t, Core.ConstType)
for isConstType to avoid dispatch issues during bootstrap where isa
may return incorrect results before the method table is fully set up.
Bootstrap now succeeds with all Compiler changes.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
