Generalize getCopyFromParts and getCopyToParts to

handle arbitrary precision integers and any number
of parts.  For example, on a 32 bit machine an i50
corresponds to two i32 parts.  getCopyToParts will
extend the i50 to an i64 then write half of the i64
to each part; getCopyFromParts will combine the two
i32 parts into an i64 then truncate the result to
i50.

llvm-svn: 47024