Simplify vector ((VCE (a cmp b ? -1 : 0)) < 0) ? c : d to just (VCE ((a cmp b) ?...

Simplify vector ((VCE (a cmp b ? -1 : 0)) < 0) ? c : d to just (VCE ((a cmp b) ? (VCE c) : (VCE d))).

When I'm working on PR112443, I notice there's some misoptimizations:
after we fold _mm{,256}_blendv_epi8/pd/ps into gimple, the backend
fails to combine it back to v{,p}blendv{v,ps,pd} since the pattern is
too complicated, so I think maybe we should hanlde it in the gimple
level.

The dump is like

  _1 = c_3(D) >= { 0, 0, 0, 0 };
  _2 = VEC_COND_EXPR <_1, { -1, -1, -1, -1 }, { 0, 0, 0, 0 }>;
  _7 = VIEW_CONVERT_EXPR<vector(32) char>(_2);
  _8 = VIEW_CONVERT_EXPR<vector(32) char>(b_6(D));
  _9 = VIEW_CONVERT_EXPR<vector(32) char>(a_5(D));
  _10 = _7 < { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  _11 = VEC_COND_EXPR <_10, _8, _9>;

It can be optimized to

  _1 = c_2(D) >= { 0, 0, 0, 0 };
  _6 = VEC_COND_EXPR <_1, b_5(D), a_4(D)>;

since _7 is either -1 or 0, the selection of _7 < 0 ? _8 : _9 should
be euqal to _1 ? b : a as long as TYPE_PRECISION of the component type
of the second VEC_COND_EXPR is less equal to the first one.
The patch add a gimple pattern to handle that.

gcc/ChangeLog:

	* match.pd (VCE (a cmp b ? -1 : 0) < 0) ? c : d ---> (VCE ((a
	cmp b) ? (VCE:c) : (VCE:d))): New gimple simplication.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512vl-blendv-3.c: New test.
	* gcc.target/i386/blendv-3.c: New test.