[SDAG] Move TRUNCATE splitting logic into a helper, and use

 it more liberally.

SplitVecOp_TRUNCATE has logic for recursively splitting oversize vectors
that need more than one round of splitting to become legal. There are many
other ISD nodes that could benefit from this logic, so factor it out and
use it for FP_TO_UINT,FP_TO_SINT,SINT_TO_FP,UINT_TO_FP and FTRUNC.

llvm-svn: 233681