Recommit r354647 and r354648 "[LegalizeTypes] When promoting the result of...

Recommit r354647 and r354648 "[LegalizeTypes] When promoting the result of EXTRACT_SUBVECTOR, also check if the input needs to be promoted. Use that to determine the element type to extract"

r354648 was a follow up to fix a regression "[X86] Add a DAG combine for (aext_vector_inreg (aext_vector_inreg X)) -> (aext_vector_inreg X) to fix a regression from my previous commit."

These were reverted in r354713 as their context depended on other patches that were reverted for a bug.

llvm-svn: 354734