SelectionDAG: Factor FP_TO_SINT lower code out of DAGLegalizer

Move the code to a helper function to allow calls from TypeLegalizer.

No functionality change intended

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>
Reviewed-by: Owen Anderson <resistor@mac.com>
llvm-svn: 212772