VU0MixVec only flush used registers

[astrelsky]

Pull Request checklist

Note: these are not necessarily requirements

• I reformatted the code with clang-format
• I checked to make sure my submission worked
• I am the author of submission or have permission from the original author
• Requires update of the PS2SDK
• Requires update of the gsKit
• Others (please specify below)

Pull Request description

I did some more research this morning and realized that the "memory" clobber flushed all of the registers. The gcc documentation recommends listing additional inputs/outputs for the accessed memory and this is the approach I took. Technically all the used registers should be listed as scratch registers so that gcc knows not to use them but I could not get it to accept any of the register names a0, $a0, etc it would only accept 0-64. AFAIK gcc cannot use the v0 vector registers directly anyway so I don't think there will be an issue. If any problems ever arise then we can fallback to the "memory" clobber.

Here is the gcc documentation https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers