It's always used in macros so that a semicolon is required after a call, just like when calling a regular function.
In your example, you have to write
__local_irq_save(1);
while
__local_irq_save(1)
would result in an error about a missing semicolon. This would not happen if the do while was not there. If it was just about scoping, a simple curly brace pair would suffice.
