[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

llvm-svn: 278002