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/lib.rs

@@ -34,7 +34,6 @@ pub mod indexset;
 pub(crate) mod ion;
 pub(crate) mod moves;
 pub(crate) mod postorder;
-pub(crate) mod ssa;
 
 #[macro_use]
 mod index;