tree-optimization/115868 - ICE with .MASK_CALL in simdclone

tree-optimization/115868 - ICE with .MASK_CALL in simdclone

The following adjusts mask recording which didn't take into account
that we can merge call arguments from two vectors like

  _50 = {vect_d_1.253_41, vect_d_1.254_43};
  _51 = VIEW_CONVERT_EXPR<unsigned char>(mask__19.257_49);
  _52 = (unsigned int) _51;
  _53 = _Z3bazd.simdclone.7 (_50, _52);
  _54 = BIT_FIELD_REF <_53, 256, 0>;
  _55 = BIT_FIELD_REF <_53, 256, 256>;

The testcase g++.dg/vect/pr68762-2.cc exercises this on x86_64 with
partial vector usage enabled and AVX512 support.

	PR tree-optimization/115868
	* tree-vect-stmts.cc (vectorizable_simd_clone_call): Correctly
	compute the number of mask copies required for vect_record_loop_mask.