make jump threading recursively simplify expressions instead of doing it just one level deep. On the testcase we go from getting this:

F1: ; preds = %T2

%F = and i1 true, %cond                         ; <i1> [#uses=1]
br i1 %F, label %X, label %Y

to a fully threaded:

F1: ; preds = %T2

br label %Y

This changes gets us to the point where we're forming (too many) switch
instructions on doug's strswitch testcase.

llvm-svn: 86646