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.

llvm-svn: 57831