Sema: reimplement runtime switch

Now supports multiple items pointing to the same body. This is a common
pattern even when using a jump table, with multiple cases pointing to
the same block of code.

In the case of a range specified, the items are moved to branches in the
else body. A future improvement may make it possible to have jump table
items as well as ranges pointing to the same block of code.

src/Module.zig

@@ -1300,6 +1300,10 @@ pub const Scope = struct {
         }
 
         pub fn addInst(block: *Block, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Ref {
+            return Air.indexToRef(try block.addInstAsIndex(inst));
+        }
+
+        pub fn addInstAsIndex(block: *Block, inst: Air.Inst) error{OutOfMemory}!Air.Inst.Index {
             const sema = block.sema;
             const gpa = sema.gpa;
 
@@ -1309,7 +1313,7 @@ pub const Scope = struct {
             const result_index = @intCast(Air.Inst.Index, sema.air_instructions.len);
             sema.air_instructions.appendAssumeCapacity(inst);
             block.instructions.appendAssumeCapacity(result_index);
-            return Air.indexToRef(result_index);
+            return result_index;
         }
     };
 };