Only record vreg definitions when fuzzing (#66)

The `vreg_def_blockparam` and `vreg_def_inst` fields on CFGInfo are only
used in `validate_ssa`, which in turn is only used in the ssagen fuzz
target.

Since these fields are never read in normal usage, initializing them is
entirely wasted effort. According to valgrind/DHAT, when running
`wasmtime compile` on the Sightglass Spidermonkey benchmark, removing
these fields saves about 100M instructions, 23k heap allocations
totalling 40MiB, and 47MiB of writes to the heap.

src/fuzzing/mod.rs

@@ -6,6 +6,7 @@
 //! Utilities for fuzzing.
 
 pub mod func;
+pub mod ssa;
 
 // Re-exports for fuzz targets.
 
@@ -21,9 +22,6 @@ pub mod moves {
 pub mod cfg {
     pub use crate::cfg::*;
 }
-pub mod ssa {
-    pub use crate::ssa::*;
-}
 pub mod ion {
     pub use crate::ion::*;
 }