Move some methods from tcx.hir() to tcx

Renamings:
- find -> opt_hir_node
- get -> hir_node
- find_by_def_id -> opt_hir_node_by_def_id
- get_by_def_id -> hir_node_by_def_id

Fix rebase changes using removed methods

Use `tcx.hir_node_by_def_id()` whenever possible in compiler

Fix clippy errors

Fix compiler

Apply suggestions from code review

Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>

Add FIXME for `tcx.hir()` returned type about its removal

Simplify with with `tcx.hir_node_by_def_id`

compiler/rustc_passes/src/dead.rs

@@ -32,7 +32,7 @@ use crate::errors::{
 // may need to be marked as live.
 fn should_explore(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     matches!(
-        tcx.hir().find_by_def_id(def_id),
+        tcx.opt_hir_node_by_def_id(def_id),
         Some(
             Node::Item(..)
                 | Node::ImplItem(..)
@@ -297,7 +297,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             // tuple struct constructor function
             let id = self.struct_constructors.get(&id).copied().unwrap_or(id);
 
-            if let Some(node) = self.tcx.hir().find_by_def_id(id) {
+            if let Some(node) = self.tcx.opt_hir_node_by_def_id(id) {
                 // When using `#[allow]` or `#[expect]` of `dead_code`, we do a QOL improvement
                 // by declaring fn calls, statics, ... within said items as live, as well as
                 // the item itself, although technically this is not the case.