[PATCH 0/2] aarch64: Fix inlining with streaming mismatches

In theory, it is possible to inline a callee function into a caller
function with incompatible streaming state, if we can prove that the
body of the callee function will behave identically in both streaming
and non-streaming mode.  Unfortunately the existing checks are far too lenient,
and can have an inconsistent impact on whether errors are reported for
mismatching always_inline functions.

This issue has cropped up in various places:
- PR 126133 was triggered by changes to the implementation of Neon intrinsics.
  The included testcase is resolved by patch 1/2, but a similar testcase
  without the explicit always_inline attribute is valid code where the same
  function might be incorrectly inlined.  This related testcase is addressed in
  patch 2/2.
- PR 124416 is a straightforward example of the bug, fixed by patch 2/2.
- Multiple uncommmitted patches have tried to lower halving-add intrinsics to
  GIMPLE, which results in the sme/arm_neon_[123].c tests failing.  There's
  nothing special about these particular intrinsics, besides them happening to
  be chosen when writing those tests.
  See https://gcc.gnu.org/pipermail/gcc-patches/2026-March/711545.html
  and https://gcc.gnu.org/pipermail/gcc-patches/2026-August/727675.html

The second patch in this series removes all of the non-testsuite code that is
changed in the first patch.  I have chosen to structure this as two separate
patches nonetheless, as I consider the first patch to be a necessary permanent
fix, while the second patch could in future be reverted as the basis for a
fixed implementation of AARCH64_IPA_SM_FIXED.

Are these patches ok for master (assuming regression testing passes)?  And ok
to backport to GCC 14-16?

Thanks,
Alice