[X86] X86DAGToDAGISel::combineIncDecVector(): call getSplatBuildVector() manually

As reported in post-commit review of r370327,
there is some case where the code crashes.

As discussed with Craig Topper, the problem is that getConstant()
internally calls getSplatBuildVector(), so we don't insert
the constant itself.

If we do that manually we're good.

llvm-svn: 371346