Adjust SwitchSources small vec size.

I did some rough measurements that show, if `SwitchSources` is
instantiated, one entry only covers ~60% of cases but two entries covers
~96% of cases.

Author: nnethercote

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -24,7 +24,7 @@ type Predecessors = IndexVec<BasicBlock, SmallVec<[BasicBlock; 4]>>;
 // `BasicBlocks::switch_sources`, which is only called by backwards analyses
 // that do `SwitchInt` handling, and we don't have any of those, not even in
 // tests. See #95120 and #94576.
-type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[u128; 1]>>;
+type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[u128; 2]>>;
 
 #[derive(Clone, Default, Debug)]
 struct Cache {