[SDAG] Fix a result chain in ExpandUnalignedLoad

On the code path in ExpandUnalignedLoad which expands an unaligned vector/fp
value in terms of a legal integer load of the same size, the ChainResult needs
to be the chain result of the integer load.

No in-tree test case is currently available.

Patch by Jan Hranac!

llvm-svn: 243956