middle-end/105049 - fix uniform_vector_p and vector CTOR gimplification

middle-end/105049 - fix uniform_vector_p and vector CTOR gimplification

We have

  return VIEW_CONVERT_EXPR<U>( VEC_PERM_EXPR < {<<< Unknown tree: compound_literal_expr
        V D.1984 = { 0 }; >>>, { 0 }} , {<<< Unknown tree: compound_literal_expr
        V D.1985 = { 0 }; >>>, { 0 }} , { 0, 0 } >  & {(short int) SAVE_EXPR <c>, (short int) SAVE_EXPR <c>});

where we gimplify the init CTORs to

  _1 = {{ 0 }, { 0 }};
  _2 = {{ 0 }, { 0 }};

instead of to vector constants.  That later runs into a bug in
uniform_vector_p which doesn't handle CTORs of vector elements
correctly.

The following adjusts uniform_vector_p to handle CTORs of vector
elements.

2022-03-25  Richard Biener  <rguenther@suse.de>

	PR middle-end/105049
	* tree.cc (uniform_vector_p): Recurse for VECTOR_CST or
	CONSTRUCTOR first elements.

	* gcc.dg/pr105049.c: New testcase.