[PATCH, PR 32824] Add an additional LIM pass to assist dead code elimination

[Fang, Changpeng]

This patch fixes bug 32824: Missed reduction vectorizer after store to global is LIM'd

Patch Description:
After store to global is LIM'ed, we need a copy propagation and dead code elimination to eliminate
the dead store in the loop, and thus make the reduction vectorizable. In this patch, I added an additional LIM
pass at the very beginning of the loop optimizer, then the dead code can be eliminated by pass_dce_loop
before the vectorizer.

With this patch, the related fortran code (refer to bug 32824 report) can also be vectorized.

I have bootstrapped gcc with the patch and compiled and spec cpu2006 and found no apparent regression
in both compilation time and spec scores.

Thanks,

Changpeng 

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