[ARM] Assert that ARMDAGToDAGISel creates valid UBFX/SBFX nodes.

[ARM] Assert that ARMDAGToDAGISel creates valid UBFX/SBFX nodes.

We don't ever check these again (unless you're using
-fno-integrated-as), so make sure the extracted bits are well-defined.

I don't think it's possible to trigger any of the assertions on trunk,
but it's difficult to prove.  (The first one depends on DAGCombine to
minimize the number of set bits in AND masks; I think the others are
mathematically impossible to hit.)

llvm-svn: 335931