In this PR we had a conversion between two integer vectors that
both had scalar integer modes.  We then tried to implement the
conversion using the scalar optab for those modes, instead of
doing the conversion elementwise.

I wondered about letting through scalar modes for single-element
vectors, but I don't have any evidence that that's useful/necessary,
so it seemed better to keep things simple.

2020-02-26  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/93843
	* optabs-tree.c (supportable_convert_operation): Reject types with
	scalar modes.

gcc/testsuite/
	PR middle-end/93843
	* gcc.dg/vect/pr93843-1.c: New test.
	* gcc.dg/vect/pr93843-2.c: Likewise.