[r17-2737 Regression] FAIL: gcc.dg/vect/vect-pr126404.c (test for excess errors) on Linux/x86_64

On Linux/x86_64,

3cd23c9fd60854cd9d160ce3b341a819025bfeb8 is the first bad commit
commit 3cd23c9fd60854cd9d160ce3b341a819025bfeb8
Author: Richard Biener <[email protected]>
Date:   Mon Jul 27 19:24:32 2026 +0200

    tree-optimization/126404 - fixup conversion detection in reductions

caused

FAIL: gcc.dg/vect/vect-pr126404.c -flto -ffat-lto-objects (internal compiler error: in vect_build_slp_tree_2, at tree-vect-slp.cc:2928)
FAIL: gcc.dg/vect/vect-pr126404.c -flto -ffat-lto-objects (test for excess errors)
FAIL: gcc.dg/vect/vect-pr126404.c (internal compiler error: in vect_build_slp_tree_2, at tree-vect-slp.cc:2928)
FAIL: gcc.dg/vect/vect-pr126404.c (test for excess errors)

with GCC configured with

../../gcc/configure --prefix=/export/users3/haochenj/src/gcc-bisect/master/master/r17-2737/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=gcc.dg/vect/vect-pr126404.c --target_board='unix{-m32\ -march=cascadelake}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="vect.exp=gcc.dg/vect/vect-pr126404.c --target_board='unix{-m64\ -march=cascadelake}'"

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