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/src/regalloc/virtregs.rs

@@ -21,6 +21,7 @@ use crate::packed_option::PackedOption;
 use crate::ref_slice::ref_slice;
 use core::cmp::Ordering;
 use core::fmt;
+use smallvec::SmallVec;
 use std::vec::Vec;
 
 /// A virtual register reference.
@@ -292,7 +293,7 @@ impl VirtRegs {
     /// Find the leader value and rank of the set containing `v`.
     /// Compress the path if needed.
     fn find(&mut self, mut val: Value) -> (Value, u32) {
-        let mut val_stack = vec![];
+        let mut val_stack = SmallVec::<[Value; 8]>::new();
         let found = loop {
             match UFEntry::decode(self.union_find[val]) {
                 UFEntry::Rank(rank) => break (val, rank),