forwprop: Ignore scalar mode vectors in simplify_vector_constructor [PR95528]

forwprop: Ignore scalar mode vectors in simplify_vector_constructor [PR95528]

As mentioned in the PR, the problem is that at least the x86 backend asumes
that the vec_unpack* and vec_pack* optabs with integral modes are for the
AVX512-ish vector masks rather than for very small vectors done in GPRs.
The only other target that seems to have a scalar mode vec_{,un}pack* optab
is aarch64 as discussed in the PR, so there is also a condition for that.
All other targets have just vector mode optabs.

2020-06-08  Jakub Jelinek  <jakub@redhat.com>

	PR target/95528
	* tree-ssa-forwprop.c (simplify_vector_constructor): Don't use
	VEC_UNPACK*_EXPR or VEC_PACK_TRUNC_EXPR with scalar modes unless the
	type is vector boolean.

	* g++.dg/opt/pr95528.C: New test.