chore: simplify decide (b = true) and variants

[kim-em]

@[simp] theorem decide_eq_true {b : Bool} : decide (b = true) = b := by cases b <;> simp
@[simp] theorem decide_eq_false {b : Bool} : decide (b = false) = !b := by cases b <;> simp
@[simp] theorem decide_true_eq {b : Bool} : decide (true = b) = b := by cases b <;> simp
@[simp] theorem decide_false_eq {b : Bool} : decide (false = b) = !b := by cases b <;> simp

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 959ad988611db082ae4a97e9b8c4390fab53352e --onto 8b8e001794e6a8b481d37f24fa77bb07797682a1. (2024-02-21 04:22:42)