[llvm] Convert as many LivePhysRegs uses to LiveRegUnits (PR #83905)

[Matt Arsenault via llvm-commits]

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@@ -313,9 +313,6 @@ MachineBasicBlock::reverse_iterator SIOptimizeExecMasking::findExecCopy(
   return E;
 }
 
-// XXX - Seems LivePhysRegs doesn't work correctly since it will incorrectly
-// report the register as unavailable because a super-register with a lane mask
-// is unavailable.
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arsenm wrote:

>From the context, I'm not sure the comment should be dropped. Maybe just should refer to LiveRegUnits 

https://github.com/llvm/llvm-project/pull/83905