[Bug tree-optimization/85478] ICE with single element vector

["krebbel at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85478

--- Comment #1 from Andreas Krebbel <krebbel at gcc dot gnu.org> ---
The testcases ICEs since r253196:

S/390: Set the preferred mode for float vectors

    gcc/ChangeLog:

    2017-09-26  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

            * config/s390/s390.c (s390_preferred_simd_mode): Return V4SFmode
            for SFmode.


with:

during RTL pass: reload
t2.cc: In member function ‘dealii::FullMatrix<number>&
dealii::FullMatrix<number>::operator=(const dealii::FullMatrix<number2>&) [with
number2 = std::complex<float>; number = std::complex<long double>]’:
t2.cc:199:3: internal compiler error: Max. number of generated reload insns per
insn is achieved (90)

   }
   ^
0x185f553 lra_constraints(bool)
        /home/andreas/gcc/gcc/lra-constraints.c:4756
0x1845459 lra(_IO_FILE*)
        /home/andreas/gcc/gcc/lra.c:2390
0x17f260b do_reload
        /home/andreas/gcc/gcc/ira.c:5440
0x17f260b execute
        /home/andreas/gcc/gcc/ira.c:5624
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With the poly-int patches the ICE is triggered during vectorization already
probably papering over the original ICE.

With the patch posted here the vectorization will not continue and does not
appear to end up in that situation anymore:

https://gcc.gnu.org/ml/gcc-patches/2018-04/msg00758.html