[GISel][CSE][NFC]: Handle mutual recursion when inserting node

GISel's CSE mechanism lazily inserts instructions into the CSE List
to improve on efficiency as well as efficacy of CSE
(for allowing partially built instructions to be fully built).

There's unfortunately a mutual recursion via
 `handleRecordedInsts -> handleRecordedInst -> insertNode-> handleRecordedInsts`.

So this change simply records that we're already draining this list so we can just bail out on the recursion.

No changes to codegen are expected as we're still draining/handling the temporary
list via pop_back and we should get the same sequence of instructions
whether we call pop_back in a loop at the top level or recursive.

https://reviews.llvm.org/D145006

reviewed by: dsanders