[r17-2139 Regression] FAIL: gfortran.dg/warn_undefined_vars_6.f90 -O (test for excess errors) on Linux/x86_64

On Linux/x86_64,

2fef81e96ea3035c6a99dd22c64f610bde40d42c is the first bad commit
commit 2fef81e96ea3035c6a99dd22c64f610bde40d42c
Author: Thomas Koenig <[email protected]>
Date:   Sat Jul 4 19:03:26 2026 +0200

    Fix two issues with unused/unset variable warnings.

caused

FAIL: gfortran.dg/warn_undefined_vars_6.f90   -O  (test for excess errors)

with GCC configured with

../../gcc/configure --prefix=/export/users3/haochenj/src/gcc-bisect/master/master/r17-2139/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="dg.exp=gfortran.dg/warn_undefined_vars_6.f90 --target_board='unix{-m32}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="dg.exp=gfortran.dg/warn_undefined_vars_6.f90 --target_board='unix{-m32\ -march=cascadelake}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="dg.exp=gfortran.dg/warn_undefined_vars_6.f90 --target_board='unix{-m64}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="dg.exp=gfortran.dg/warn_undefined_vars_6.f90 --target_board='unix{-m64\ -march=cascadelake}'"

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