RFR(S): 8028471: PPC64 (part 215): opto: Extend ImplicitNullCheck optimization.

[Lindenmaier, Goetz]

Hi,

ImplicitNullChecks did not work on platforms where the zero
page is only write protected.

This change adds os property 'zero_page_read_protected' and extends
the ImplicitNullCheck optimization to consider only stores if
this property is not true. If a decoded compressed oop will access the
guard page before the heap, Loads work again.
This is needed on AIX, where the page at address '0' is only write-protected.

Please review and test this change:
http://cr.openjdk.java.net/~goetz/webrevs/8028471-0-zpage/

I tagged this S, as a majority of the patches apply to the ppc platform files.

Best regards,
  Goetz.
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