teach tblgen to be more aggressive when factoring CheckType nodes.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97908 91177308-0d34-0410-b5e6-96231b3b80d8