Add FileCheck annotations to retag mir-opt test

[AprilNEA]

Part of #116971.

Add FileCheck annotations to tests/mir-opt/retag.rs, removing the // skip-filecheck directive.

The added CHECK directives verify that the AddRetag pass correctly inserts Retag statements:

• At function entry for self and reference parameters (Retag([fn entry] _1), Retag([fn entry] _2))
• Uses CHECK-NEXT to verify Retag statements appear immediately after bb0: {
• After reborrows for shared reference return values (Retag(_0))
• For closures (main::{closure#0}) with proper entry retags
• For array_casts function (testing retags through &raw)
• For box_to_raw_mut function with [raw] retag (Retag([raw] _0))

r? @cjgillot

[rustbot]

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