[TableGen][ISel] When factoring ScopeMatcher, if the child of the ScopeMatcher...

[TableGen][ISel] When factoring ScopeMatcher, if the child of the ScopeMatcher we're working on is also a ScopeMatcher, merge all its children into the one we're working on.

There were several cases in X86 where we were unable to fully factor a ScopeMatcher but created nested ScopeMatchers for some portions of it. Then we created a SwitchType that split it up and further factored it so that we ended up with something like this:

SwitchType
  Scope
    Scope
      Sequence of matchers
      Some other sequence of matchers
    EndScope
    Another sequence of matchers
  EndScope
...Next type

This change turns it into this:

SwitchType
  Scope
    Sequence of matchers
    Some other sequence of matchers
    Another sequence of matchers
  EndScope
...Next type

Several other in-tree targets had similar nested scopes like this. Overall this doesn't save many bytes, but makes the isel output a little more regular.

llvm-svn: 287624