[AArch64] PR28877: Don't assume we're running after legalization when
creating vcvtfp2fxs

Summary:
The DAG combine transformation that was generating the
aarch64_neon_vcvtfp2fxs node was assuming that all inputs where legal
and wasn't accounting that the input could be a v4f64 if we're trying
to do the transformation before legalization. We now bail out in this
case.

All illegal types besides v4f64 were already rejected.

Fixes https://llvm.org/bugs/show_bug.cgi?id=28877

Reviewers: jmolloy

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D23261