[dev.ssa] cmd/compile/internal/ssa: large object load/store vs. vardef

[dev.ssa] cmd/compile/internal/ssa: large object load/store vs. vardef

VarDef declarations are getting in the way of rewriting load/store
pairs into moves.  This change fixes that, albeit in a really hacky way.
Better options would be appreciated.

Increases coverage during make.bash from 67% to 71%.

Change-Id: I336e967687e2238c7d0d64e3b37132a731ad15c3
Reviewed-on: https://go-review.googlesource.com/14347
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>