[DAGCombiner] Add support for TRUNCATE + FP_EXTEND vector constant folding

This patch adds supports for the vector constant folding of TRUNCATE and FP_EXTEND instructions and tidies up the SINT_TO_FP and UINT_TO_FP instructions to match.

It also moves the vector constant folding for the FNEG and FABS instructions to use the DAG.getNode() functionality like the other unary instructions.

Differential Revision: http://reviews.llvm.org/D8593

llvm-svn: 233224