[Issue 21490] Front end detection logic for SSE4.1 is failing when using -O

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21490

--- Comment #1 from Walter Bright <bugzilla at digitalmars.com> ---
Test case clipped from dmd/test/runnable/testxmm.d:

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import core.simd;

void test2e()
{
    int4 v1, v2 = 1, v3 = 1;
    v1 = v2 + v3;
    static if (__traits(compiles, { v1 = v2 * v3; })) // SSE4.1
        v1 = v2 * v3;
}

int main() { test2e(); return 0; }
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This will seg fault on illegal PMULLD instruction if running on a machine
without SSE4.1

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