refactor: replace `isImplicitReducible` with `Meta.isInstance` in `shouldInline`

This PR replaces the `isImplicitReducible` check in `shouldInline` with
`Meta.isInstance`. At the base phase, we avoid inlining instances tagged
with `[inline]/[always_inline]/[inline_if_reduce]` because their local
functions will be lambda lifted during the base phase. Inlining them
prematurely creates extra work for the lambda lifter and produces many
lambda-lifted closures, which is particularly problematic for the IR
interpreter (causing C++ stack overflows through stub closures).

The previous `isImplicitReducible` check did not capture the original
intent: some `instanceReducible` declarations are not instances. Using
`Meta.isInstance` correctly targets only actual instances. This is
scope-safe because LCNF compilation runs at `addDecl` time, before any
scope changes that could affect instance visibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>