[LegalizeVectorOps] After custom legalizing an extending load or a truncating...

[LegalizeVectorOps] After custom legalizing an extending load or a truncating store, make sure the custom code is also legal.

For example, on X86 we emit a sign_extend_vector_inreg from LowerLoad and without sse4.1 this node will need further legalization. Previously this sign_extend_vector_inreg was being custom lowered during DAG legalization instead of vector op legalization.

Unfortunately, this doesn't seem to matter for the output of any existing lit tests.

llvm-svn: 347094