[PATCH] libstdc++: Add masked ++/-- implementation for sizeof < 16

[Matthias Kretz]

And one more to fix follow-up / remaining failures. Tested on x86_64-linux-
gnu.

OK for trunk and 13?

------------ 8< --------------

This resolves further failures (-Wreturn-type warnings) and test
failures for where-* tests targeting AVX-512.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd_x86.h (_S_masked_unary):
	Cast inputs < 16 bytes to 16 byte vectors before calling the
	right subtraction builtin. Before returning, truncate to the
	return vector type.
---
 .../include/experimental/bits/simd_x86.h      | 24 +++++++++++--------
 1 file changed, 14 insertions(+), 10 deletions(-)


--
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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 stdₓ::simd
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