cmd/compile: mark modify ops as both read and write
If the modify ops operate on a variable, we should tell the liveness
pass that the variable is still live before the instruction.
This looks like a bug, but I don't think there's any way to trigger
it at the moment. It only matters for pointer-containing values, and
the modify ops don't normally work on pointers. Even when I reach for
unsafe.Pointer tricks, I can't get ADDLmodify to work on pointers, as
there's always a Convert or VarDef preventing the coalescing.
TL;DR I can't figure out a test for this. But we should probably
fix it anyway.