[PR tree-optimization/84047] missing -Warray-bounds on an out-of-bounds index

[Aldy Hernandez]

Hi!

[Note: Jakub has mentioned that missing -Warray-bounds regressions 
should be punted to GCC 9.  I think this particular one is easy 
pickings, but if this and/or the rest of the -Warray-bounds regressions 
should be marked as GCC 9 material, please let me know so we can adjust 
all relevant PRs.]

This is a -Warray-bounds regression that happens because the IL now has 
an MEM_REF instead on ARRAY_REF.

Previously we had an ARRAY_REF we could diagnose:

   D.2720_5 = "12345678"[1073741824];

But now this is represented as:

   _1 = MEM[(const char *)"12345678" + 1073741824B];

I think we can just allow check_array_bounds() to handle MEM_REF's and 
everything should just work.

The attached patch fixes both regressions mentioned in the PR.

Tested on x86-64 Linux.

OK?
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