veclower: Don't ICE on .VEC_CONVERT calls with no lhs [PR96426]

veclower: Don't ICE on .VEC_CONVERT calls with no lhs [PR96426]

.VEC_CONVERT is a const internal call, so normally if the lhs is not used,
we'd DCE it far before getting to veclower, but with -O0 (or perhaps
-fno-tree-dce and some other -fno-* options) it can happen.
But as the internal fn needs the lhs to know the type to which the
conversion is done (and I think that is a reasonable representation, having
some magic another argument and having to create constants with that type
looks overkill to me), we just should DCE those calls ourselves.
During veclower, we can't really remove insns, as the callers would be
upset, so this just replaces it with a GIMPLE_NOP.

2020-08-04  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/96426
	* tree-vect-generic.c (expand_vector_conversion): Replace .VEC_CONVERT
	call with GIMPLE_NOP if there is no lhs.

	* gcc.c-torture/compile/pr96426.c: New test.