Don't try to extract an i32 from an f64. This

getCopyToParts problem was noticed by the new
LegalizeTypes infrastructure.  In order to avoid
this kind of thing in the future I've added a
check that EXTRACT_ELEMENT is only used with
integers.  Once LegalizeTypes is up and running
most likely BUILD_PAIR and EXTRACT_ELEMENT can
be removed, in favour of using apints instead.

llvm-svn: 48294