Fix for https://bugs.llvm.org/show_bug.cgi?id=42760. A tBR_JTr instruction is duplicated by tail duplication, which results in the same jumptable with the same label being emitted twice.
Fix this by marking tBR_JTr as not duplicable. The corresponding ARM instructions (BR_JTr, BR_JTm_i12, BR_JTm_rs and BR_JTadd) are already marked as not duplicable.
Not sure whether the test-case adds any value, it's a fairly ugly bugpoint reduction of the original issue.