SI-7185 Avoid NPE in TreeInfo.isExprSafeToInline

We got there typechecking code with a redundant
layer of Block.

We can't express that in source code, so we test
this with manual tree construction and with XML
literals, which as reported produce such trees.
Merge pull request #2175 from retronym/ticket/7185

SI-7185 Avoid NPE in TreeInfo.isExprSafeToInline