teach tblgen to be more aggressive when factoring CheckType nodes.

Now it will factor things like this:

CheckType i32
  ...
CheckOpcode ISD::AND
  CheckType i64
  ...

into:

SwitchType:
  i32: ...
  i64:
    CheckOpcode ISD::AND
    ...

This shrinks hte table by a few bytes, nothing spectacular.

llvm-svn: 97908