[AArch64][SVE] Fix isel failure for FP-extending loads

DAGCombiner tries to combine a (fpext (load)) to (fround (extload))
but SVE has no FP-extending loads. By marking these as expand,
the combine no longer happens.

This also fixes a similar issue for fptrunc, where the source type
is not a legal type.

Reviewed By: bsmith, kmclaughlin

Differential Revision: https://reviews.llvm.org/D102053