interp: add support for switch statement

A switch statement is not normally emitted by the compiler package, but
LLVM function passes may convert a series of if/else pairs to a switch
statement. A future change will run function passes in the package
compile phase, so the interp package (which is also run after all
modules are merged together) will need to deal with these new switch
statements.

interp/testdata/basic.out.ll

@@ -25,6 +25,8 @@ entry:
   store i16 5, i16* @main.exposedValue1
   call void @modifyExternal(i32* bitcast (void ()* @willModifyGlobal to i32*))
   store i16 7, i16* @main.exposedValue2
+  call void @runtime.printint64(i64 6)
+  call void @runtime.printint64(i64 -1)
   ret void
 }
 
@@ -44,3 +46,24 @@ entry:
   store i16 8, i16* @main.exposedValue2
   ret void
 }
+
+define i64 @testSwitch(i64 %val) local_unnamed_addr {
+entry:
+  switch i64 %val, label %otherwise [
+    i64 0, label %zero
+    i64 1, label %one
+    i64 2, label %two
+  ]
+
+zero:                                             ; preds = %entry
+  ret i64 5
+
+one:                                              ; preds = %entry
+  ret i64 6
+
+two:                                              ; preds = %entry
+  ret i64 7
+
+otherwise:                                        ; preds = %entry
+  ret i64 -1
+}