[r17-1485 Regression] FAIL: g++.dg/vect/slp-pr98855.cc -std=c++26 scan-tree-dump slp1 "not vectorized: vectorization is not profitable" on Linux/x86_64

On Linux/x86_64,

9f8409f2e2c28f64bf6a584bc81afcae0f084785 is the first bad commit
commit 9f8409f2e2c28f64bf6a584bc81afcae0f084785
Author: Zhongyao Chen <[email protected]>
Date:   Wed Jun 3 20:44:59 2026 +0800

    vect: Avoid external fallback while operand swap retry is possible

caused

FAIL: g++.dg/vect/slp-pr98855.cc  -std=c++26  scan-tree-dump-not slp1 "vectorizing stmts using SLP"
FAIL: g++.dg/vect/slp-pr98855.cc  -std=c++26  scan-tree-dump slp1 "not vectorized: vectorization is not profitable"

with GCC configured with

../../gcc/configure --prefix=/export/users3/haochenj/src/gcc-bisect/master/master/r17-1485/usr --enable-clocale=gnu --with-system-zlib --with-demangler-in-ld --with-fpmath=sse --enable-languages=c,c++,fortran --enable-cet --without-isl --enable-libmpx x86_64-linux --disable-bootstrap

To reproduce:

$ cd {build_dir}/gcc && make check RUNTESTFLAGS="vect.exp=g++.dg/vect/slp-pr98855.cc --target_board='unix{-m64\ -march=cascadelake}'"

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(If you met problems with cascadelake related, disabling AVX512F in command line might save that.)
(However, please make sure that there is no potential problems with AVX512.)