[dev.ssa] cmd/compile: exposed do-log boolean to reduce allocations

From memory profiling, about 3% reduction in allocation count.

Change-Id: I4b662d55b8a94fe724759a2b22f05a08d0bf40f8
Reviewed-on: https://go-review.googlesource.com/19103
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index 7e6e544..e338c44 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -147,6 +147,7 @@
 }
 
 func (v *Value) Logf(msg string, args ...interface{}) { v.Block.Logf(msg, args...) }
+func (v *Value) Log() bool                            { return v.Block.Log() }
 func (v *Value) Fatalf(msg string, args ...interface{}) {
 	v.Block.Func.Config.Fatalf(v.Line, msg, args...)
 }