Add custom lowering for `BR_JT` DAG nodes to the `brx.idx` PTX
instruction ([PTX ISA 9.7.13.4. Control Flow Instructions: brx.idx]
(https://docs.nvidia.com/cuda/parallel-thread-execution/#control-flow-instructions-brx-idx)).
Depending on the heuristics in DAG selection, `switch` statements may
now be lowered using `brx.idx`.
Note: this fixes the previous issue in #102400 by adding the isBarrier
attribute to BRX_END