[m-dev.] diff: bug fix for `-O0 --warn-duplicate-calls'

[Simon TAYLOR]

> Simon, can you please review this change?
> 
> compiler/simplify.m:
> 	Fix a bug: the `--warn-duplicate-calls' option did not work
> 	unless the `--optimize-duplicate-calls' option was also enabled.
> 	This caused the test case tests/warnings/duplicate_call.m to fail
> 	when MCFLAGS was set to `-O0'.

Thanks Fergus.