VirtRegs::find: use SmallVec instead of Vec for val_stack.

Pushing on the `val_stack` vector is CL's biggest source of calls to
malloc/realloc/free, by some margin.  It accounts for about 27.7% of all heap
blocks allocated when compiling wasm_lua_binarytrees.  This change removes
pretty much all dynamic allocation by changing to a SmallVec<[Value; 8]>
instead.  A fixed size of 4 gets all the gains to be had, in testing, so 8
gives some safety margin and is harmless from a stack-use perspective: 8
Values will occupy 32 bytes.

As a bonus, this change also reduces the compiler's dynamic instruction count
by about 0.5%.

cranelift/codegen/Cargo.toml

@@ -21,6 +21,7 @@ hashmap_core = { version = "0.1.9", optional = true }
 target-lexicon = "0.8.1"
 log = { version = "0.4.6", default-features = false }
 serde = { version = "1.0.94", features = ["derive"], optional = true }
+smallvec = { version = "0.6.10" }
 # It is a goal of the cranelift-codegen crate to have minimal external dependencies.
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be