For various statements the spec states:
Return NormalCompletion(empty).
In those cases we have been returning undefined so far, which is
incorrect.
In other cases it states:
Return Completion(UpdateEmpty(stmtCompletion, undefined)).
Which essentially means a statement is evaluated and its completion
value returned if non-empty, and undefined otherwise.
While not actually noticeable in normal scripts as the VM's "last value"
can't be accessed from JS code directly (with the exception of eval(),
see below), it provided an inconsistent experience in the REPL:
> if (true) 42;
42
> if (true) { 42; }
undefined
This also fixes the case where eval() would return undefined if the last
executed statement is not a value-producing one:
eval("1;;;;;")
eval("1;{}")
eval("1;var a;")
As a consequence of the changes outlined above, these now all correctly
return 1.
See https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation,
"NOTE 2".
Fixes#3609.
