[Patch] OpenACC: Fix reduction tree-sharing issue [PR106982]

[Tobias Burnus]

This fixes a tree-sharing ICE. It seems as if all unshare_expr
I added were required in this case. The first long testcase is
based on the real testcase from the OpenACC testsuite, the second
one is what reduction produced - but I thought some nested reduction
might be interesting as well; hence, I included both tests.


Bootstrapped and regtested on x86-64-gnu-linux w/o offloading.
OK for mainline and GCC 12?

(It gives an ICE with GCC 10 but not with GCC 9; thus,
more regression-fix backporting would be possible,
if someone cares.)

Tobias


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