Avoid constructing switch sources unless necessary

Switch sources are used by backward analysis with a custom switch int
edge effects, but are otherwise unnecessarily computed.

Delay the computation until we know that switch sources are indeed
required and avoid the computation otherwise.

compiler/rustc_middle/src/mir/mod.rs

@@ -580,6 +580,8 @@ impl<'tcx> Body<'tcx> {
         self.predecessor_cache.compute(&self.basic_blocks)
     }
 
+    /// `body.switch_sources()[target][switch]` returns a list of switch values
+    /// that lead to a `target` block from a `switch` block.
     #[inline]
     pub fn switch_sources(&self) -> &SwitchSources {
         self.switch_source_cache.compute(&self.basic_blocks)