If the type legalizer actually legalized anything

If the type legalizer actually legalized anything
(this doesn't happen that often, since most code
does not use illegal types) then follow it by a
DAG combiner run that is allowed to generate
illegal operations but not illegal types.  I didn't
modify the target combiner code to distinguish like
this between illegal operations and illegal types,
so it will not produce illegal operations as well
as not producing illegal types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59960 91177308-0d34-0410-b5e6-96231b3b80d8