Fix unused variable warnings by using n_mulhi

[user202729]

Fix #2655

The transformations are clearly safe, and the warnings are gone on my machine.

[vneiger]

To better understand what this solves and why a solution is needed, could you please give more details on the context in which these pieces of code triggered this warning? Is this when compiling flint itself? If yes, on what kind of system/compiler/cpu?

Some comments:

• There is already a function n_mulhi in ulong_extras.h, which has the same behaviour as your macro umulhigh_pmm. It's not clear to me that there is a good reason for both to co-exist.
• There is a macro FLINT_SET_BUT_UNUSED in flint.h, which should suppress this specific warning (but maybe not not for all possible configurations of compiler/system/etc).

[albinahlback]

I believe it is being triggered for at least all Clang compilers. The only reason why some people never see these warnings is because we force GCC to use assembly, where GCC assumes that the variables are being used.

[user202729]

• I didn't know about n_mulhi, should be better now.
• Actually GCC should know the first 2 arguments are output-only; however there's no way to invoke an __asm__ while omitting the output operands. At least you need to declare dummy variables. (As far as I know)

[fredrik-johansson]

Thanks!