Fix PR94043 by making vect_live_op generate lc-phi

As PR94043 shows, my commit r10-4524 exposed one issue in
vectorizable_live_operation, which inserts one extra BB
before the single exit, leading unexpected operand expansion
and unexpected loop depth assertion.  As Richi suggested,
this patch is to teach vectorizable_live_operation to
generate loop closed phi for vec_lhs, it looks like:
     loop;
     # lhs' = PHI <lhs>
=>
     loop;
     # vec_lhs' = PHI <vec_lhs>
     new_tree = BIT_FIELD_REF <vec_lhs', ...>;
     lhs' = new_tree;

I noticed that there are some SLP cases that have same lhs
and vec_lhs but different offsets, which can make us have
more PHIs for the same vec_lhs there.  But I think it would
be fine since only one of them is actually live, the others
should be eliminated by the following dce.  So the patch
doesn't check whether there is one phi for vec_lhs, just
create one directly instead.

Bootstrapped/regtested on powerpc64le-linux-gnu (LE) P8.

2020-04-01  Kewen Lin  <linkw@gcc.gnu.org>

gcc/ChangeLog

    PR tree-optimization/94043
    * tree-vect-loop.c (vectorizable_live_operation): Generate loop-closed
    phi for vec_lhs and use it for lane extraction.

gcc/testsuite/ChangeLog

    PR tree-optimization/94043
    * gfortran.dg/graphite/vect-pr94043.f90: New test.

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