Use SmallVec in SwitchTargets

This allows building common SwitchTargets (eg. for `if`s) without
allocation.

compiler/rustc_mir/src/transform/simplify_comparison_integral.rs

@@ -227,5 +227,5 @@ struct OptimizationInfo<'tcx> {
     /// Either Eq or Ne
     op: BinOp,
     /// Current targets used in the switch
-    targets: SwitchTargets<'tcx>,
+    targets: SwitchTargets,
 }