PR rtl-optimization/15248 -- semi-latent reload bug

[Jeffrey A Law <law at redhat dot com>]

As mentioned in the PR, when we create equivalences between a pseudo
and read-only memory we run the risk of emitting an instruction which
attempts to store into the read-only memory location.  That is clearly
bad since storing into a read-only memory location will typically
generate a segfault.

I pondered trying to remove the offending assignments, but came to the
conclusion that it could not easily be done, largely because new
offending assignments can emerge as a result of spilling.

This patch takes the more conservative approach (and the same approach
that was used for 3.3 and 3.4), namely ignoring equivalences for
readonly memory locations.  This can create more spills than is 
necessary, but it is clearly safe and this situation is quite rare,
so the performance impact ought to be minimal.

This has been bootstrapped and regression tested on i686-pc-linux-gnu.
It has also been bootstrapped and regression tested in conjunction with
the pending jump threading changes which exposed this latent bug.

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