[llvm] [Hexagon] Use LiveRegUnits (PR #84112)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

There was a mistake in my comment: `IsLive` is equivalent to `!LPR.available`.  Please restore the previous change and apply this correction.

https://github.com/llvm/llvm-project/pull/84112