Fix PR94443 with gsi_insert_seq_before [PR94443]

Fix PR94443 with gsi_insert_seq_before [PR94443]

This patch is to fix the stupid mistake by using
gsi_insert_seq_before instead of gsi_insert_before.

BTW, the regression testing on one x86_64 machine from CFarm is
unable to reveal it (I guess due to native arch sandybridge?), so I
specified additional option -march=znver2 and verified the coverage.

Bootstrapped/regtested on powerpc64le-linux-gnu (P9) and
x86_64-pc-linux-gnu, also verified the fail cases in related PRs.

2020-04-03  Kewen Lin  <linkw@gcc.gnu.org>

gcc/
    PR tree-optimization/94443
    * tree-vect-loop.c (vectorizable_live_operation): Use
    gsi_insert_seq_before to replace gsi_insert_before.

gcc/testsuite/
    PR tree-optimization/94443
    * gcc.dg/vect/pr94443.c: New test.