Have X86 custom lowering for LegalizeTypes use

Have X86 custom lowering for LegalizeTypes use
LowerOperation if it doesn't know what else to do.
This methods should probably be factorized some,
but this is good enough for the moment.  Have
LowerATOMIC_BINARY_64 use EXTRACT_ELEMENT rather
than assuming the operand is a BUILD_PAIR (if it
is then getNode will automagically simplify the
EXTRACT_ELEMENT).  This way LowerATOMIC_BINARY_64
usable from LegalizeTypes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57831 91177308-0d34-0410-b5e6-96231b3b80d8