compiler: Don't mark SingleUseConsts MIR pass as "required for soundness"

Because:
* Something like it did not exist before PR 107404
* That it is not run our mir-opt-level 0 indicates that it is not
  required for soundness
* Its `MirPass::can_be_overridden()` is unchanged and thus returns true,
  indicating that it is not a required MIR pass.
* No test fails in PR 151426 that stops enabling by default in non-optimized builds

As can be seen from the updated test `tests/mir-opt/optimize_none.rs`,
this means that `#[optimize(none)]` functions become even less
optimized. As expected and as desired.

Author: Enselic

compiler/rustc_mir_transform/src/single_use_consts.rs

@@ -85,7 +85,7 @@ impl<'tcx> crate::MirPass<'tcx> for SingleUseConsts {
     }
 
     fn is_required(&self) -> bool {
-        true
+        false
     }
 }
 

tests/mir-opt/optimize_none.rs

@@ -15,13 +15,14 @@ pub fn add_noopt() -> i32 {
 #[optimize(none)]
 pub fn const_branch() -> i32 {
     // CHECK-LABEL: fn const_branch(
-    // CHECK: switchInt(const true) -> [0: [[FALSE:bb[0-9]+]], otherwise: [[TRUE:bb[0-9]+]]];
+    // CHECK: [[BOOL:_[0-9]+]] = const true;
+    // CHECK: switchInt(move [[BOOL]]) -> [0: [[BB_FALSE:bb[0-9]+]], otherwise: [[BB_TRUE:bb[0-9]+]]];
     // CHECK-NEXT: }
-    // CHECK: [[FALSE]]: {
+    // CHECK: [[BB_FALSE]]: {
     // CHECK-NEXT: _0 = const 0
     // CHECK-NEXT: goto
     // CHECK-NEXT: }
-    // CHECK: [[TRUE]]: {
+    // CHECK: [[BB_TRUE]]: {
     // CHECK-NEXT: _0 = const 1
     // CHECK-NEXT: goto
     // CHECK-NEXT: }