RFA: make PLUS commutative by default

[Joern RENNECKE]

Jeffrey A Law wrote:

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>>It is not just PA any more.  IIRC ia64 has the same issue too.
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>And the mn10x ports IIRC as well.
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The mn10200 port is no longer in mainline.  The mn10300
GO_IF_LEGITIMATE_ADDRESS is implemented with a function that
rejects base reg + index reg addressing (apparently for this very reason).
So apparently we don't need to worry about the mn10x ports now.