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.

Change-Id: I971c62616dec51a33788b7634e6478e1bfcd6260
Reviewed-on: https://go-review.googlesource.com/112157Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>