Eliminate dead stores.  Dead stores are those which are
unconditionally followed by another store to the same location, with
no intervening load.

Just a simple intra-block implementation for now.

Change-Id: I2bf54e3a342608fc4e01edbe1b429e83f24764ab
Reviewed-on: https://go-review.googlesource.com/10386Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>