[Bug target/62191] extra shift generated for vector integer division by constant 2

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62191

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
  vector(4) int _4;
  _4 = x_1(D) >> 31;
  _5 = VIEW_CONVERT_EXPR<vector(4) unsigned int>(_4);
  _6 = _5 >> 31;

(lshiftrt (ashiftrt X N) 31) is just (lshiftrt X 31)

The scalar case is handled in combine, probably by simplify_shift_const_1. We
could tweak it to handle vectors, but mostly we should have a similar
transformation in match.pd, and make sure it handles both scalars and vectors.