svolume: Mark channel parameter as earlyclobber

For all our MMX/SSE code, we use a temporary channel variable, assigned to the DI register, which is zero'ed as the very first operation in the inline assembly code, before any other code is run.

With GCC 9.1, while using -O2, the DI register is also used for the input operand. This is perfectly legal, but causes our code to become incorrect because the output operand that is assigned to DI is not explicitly marked as being clobbered before inputs are read.

This change fixes the problem by adding an earlyclobber annotation (&) to the DI output argument.