test: Avoid -ffinite-math-only on floating point comparisons

[iv-m]

With -ffinite-math-only (implied by -ffast-math and -Ofast), GCC considers all comparisons against infinities to be false and can optimize them away. This breaks several tests that rely on infinities to be compared correctly. To avoid this, we add GCC-specific optimize attribute that disables -ffinite-math-only optimization for floating point comarisions used in assertions.

[mr-c]

-Ofast produces errors with various GCC versions; perhaps they are compiler errors, perhaps they are issues that SIMDe can workaround.

Regardless, they need addressing before we merge; unless -ffast-math is sufficient?

[iv-m]

The idea of commit test: Avoid -ffinite-math-only on floating point comparisons was to fix a very specific case of failing tests: when an assertion needs to compare two infinities (one returned from the function under test, and one from the expected response), but GCC assumed such comparison to be false because of -ffinite-math-only.

Since -ffast-math implies -ffinite-math-only, -ffast-math is enough to trigger this failure mode. Also, since this commit addresses a specific issue triggered by -ffinite-math-only, it probably won't help with any tests that are passing with -ffast-math but failing with -Ofast or when -O3 is added.

What it can do is to allow to unskip some tests that are currently skipped when SIMDE_FAST_MATH is set, which seems to be quite a lot of tests.

[mr-c]

@iv-m Great! Can you revert the change to use -Ofast where that failed, and then adjust the test skips where we can benefit?

[iv-m]

@iv-m Great! Can you revert the change to use -Ofast where that failed, and then adjust the test skips where we can benefit?

Well, I won't promise I'll review every entrance of SIMDE_FAST_MATH in the tests, but I think I can easily enable several tests (a couple of hundred #ifs from my rough initial estimate) to demonstrate that this approach works.

And I think since this change affects only some GCC-like compilers, these tests should stay disabled for all the rest of the compilers, and so another feature-test macro is needed for use in place of SIMDE_FAST_MATH. For example:

#if defined(SIMDE_FAST_MATH)
#define SIMDE_TEST_EQUALTITY_WORKS_FOR_INFINITIES
#endif
#if defined(SIMDE_FAST_MATH) && defined(HEDLEY_GCC_VERSION) && HEDLEY_GCC_VERSION_CHECK(4,4,0)
/* For these GCC versions, `-ffinite-math-only` is disabled for
 * the functions that are used in assertions */
#define SIMDE_TEST_EQUALTITY_WORKS_FOR_INFINITIES
#endif

SIMDE_TEST_EQUALTITY_WORKS_FOR_INFINITIES is somewhat long name. Maybe a negative macro, e.g. SIMDE_TEST_INFINITE_EQ_BROKEN, is better? Maybe you have better naming suggestions?