review for 6726504 (S): handle do_ifxxx calls in parser more uniformly

[Chuck Rasbold]

http://webrev.invokedynamic.info/rasbold/6726504/

6726504: handle do_ifxxx calls in parser more uniformly
Summary: make do_ifnull() handling similar to do_if()
Reviewed-by: jrose

Some cleanups prompted by the recent fix to 6707044:

Pop ifnull/ifnonnull stack operands in the main switch
statement.

Factor call to maybe_add_safepoint() into do_if().

Remove some dead node construction.