[RISCV] Simplify computation of VarArgsSaveSize. NFC (#74209)

The computation we use for computing the size already returns 0 when all
registers are allocated. We don't need an if to set it to 0.

Use the size being 0 to check for whether we need to spill registers or
not.

I have another change I want to make to this code, but this change
seemed to stand on its own. I left the curly braces since I need them
for the other change.