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

[Matt Arsenault via llvm-commits]

================
@@ -313,7 +313,7 @@ MachineBasicBlock::reverse_iterator SIOptimizeExecMasking::findExecCopy(
   return E;
 }
 
-// XXX - Seems LivePhysRegs doesn't work correctly since it will incorrectly
+// XXX - Seems LiveRegUnits doesn't work correctly since it will incorrectly
 // report the register as unavailable because a super-register with a lane mask
 // is unavailable.
----------------
arsenm wrote:

This comment is likely inaccurate now 

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