[Bug rtl-optimization/118502] New: Add shrink wrapping testcase for vector::push_back

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118502

            Bug ID: 118502
           Summary: Add shrink wrapping testcase for
                    vector<int>::push_back
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, needs-testcase
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

GCC 15 started to shrink wrap:
```
#include <cstdint>
#include <vector>

void push_back(std::vector<int>& xs, uint8_t x) {
    xs.push_back(x);
}
```

on both aarch64 and x86_64.  We should add a testcase to make sure we don't
regress.