[llvm] [ARM] Fix outlining suitability of tBLXr instructions (PR #206104)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Is there a reason to restrict the registers for tTAILJMPr in the first place?

TCRETURNri needs restrictions because otherwise epilogue lowering blows up, but by the time we emit tTAILJMPr, we're past that; if we can somehow prove it's safe to use a different register, that's fine, I think.

https://github.com/llvm/llvm-project/pull/206104