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>
R600: Implement float to long/ulong

Use alg. from LegalizeDAG.cpp
Move Expand setting to SIISellowering

v2: Extend existing tests instead of creating new ones
v3: use separate LowerFPTOSINT function
v4: use TargetLowering::expandFP_TO_SINT
    add comment about using FP_TO_SINT for uints

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>