Use the FSA instead of malloc in MVM_jit_expr_tree_build

The Fixed Size Allocator should be slightly faster than malloc for the
small allocations we need, and possibly much faster than free, because we
know the size of the memory we are deallocating.

src/jit/expr.c

@@ -707,13 +707,15 @@ MVMJitExprTree * MVM_jit_expr_tree_build(MVMThreadContext *tc, MVMJitGraph *jg,
         MVMuint16 operands_needed = MAX(2, ins->info->num_operands); /* At least 2 for inc_i hack */
         if (operands_needed > operands_allocated) {
             /* Round up to an even number - we're allocating an array of 4 byte
-               integers, so assume malloc will align (at least) to 8 bytes, and
-               hence avoid some "churn" */
+               integers, whilst the FSA will align (at least) to 8 bytes. Hence
+               avoid some "churn" */
             if (operands_needed & 1) {
                 ++operands_needed;
             }
-            MVM_free(operands);
-            operands = MVM_malloc(operands_needed * sizeof(MVMint32));
+            if (operands) {
+                MVM_fixed_size_free(tc, tc->instance->fsa, operands_allocated * sizeof(MVMint32), operands);
+            }
+            operands = MVM_fixed_size_alloc(tc, tc->instance->fsa, operands_needed * sizeof(MVMint32));
             operands_allocated = operands_needed;
         }
         MVMSpeshAnn *ann;
@@ -914,7 +916,7 @@ MVMJitExprTree * MVM_jit_expr_tree_build(MVMThreadContext *tc, MVMJitGraph *jg,
         tree = NULL;
     }
     MVM_free(values);
-    MVM_free(operands);
+    MVM_fixed_size_free(tc, tc->instance->fsa, operands_allocated * sizeof(MVMint32), operands);
     return tree;
 }