[Issue 21490] New: 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

          Issue ID: 21490
           Summary: Front end detection logic for SSE4.1 is failing when
                    using -O
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Adding -O to test/runnable/testxmm.d results in a seg fault when testxmm is run
because a PMULLD instruction is generated as a result of:

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

succeeding, yet my 64 bit linux box does not support that instruction. Weirdly,
this only goes awry with -O.

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