lavapipe: fix fsum with swizzle

We can do stuff like this:

vec1 32 ssa_207 = fsum3 ssa_209.xxx

In this case, we'd end up not swizzling in get_alu_src, and reading components out-of-bounds, which LLVM isn't very happy about, and thus takes punitive actions, in the form of a segfault.

We don't want that, and we already know from the opcode what the component counts should be here.