[COMMITTED] tree-optimization/103022 - Don't calculate new values when using the private context, callback.

[Andrew MacLeod]

When using the private communication method I introduced for fold_stmt 
to acquire context, we are suppose to be using the cache in read-only 
mode.  I didn't query it in that mode, so it was accidentally triggering 
another lookup before finishing the original query, and that triggered 
the safety trap.  This patch makes sure we make the request in read-only 
mode .

Bootstraps with no regressions on x86_64-pc-linux-gnu.  Pushed.

Andrew

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