[GC] Add struct.new and start to test interesting execution (#3433)

With struct.new read/write support, we can start to do interesting
things! This adds a test of creating a struct and seeing that references
behave like references, that is, if we write to the value X refers to, and
if Y refers to the same thing, when reading from Y's value we see the
change as well.

The test is run through all of -O1, which uncovered a minor issue in
Precompute: We can't try to precompute a reference type, as we can't
replace a reference with a value.

Note btw that the test shows the optimizer properly running
CoalesceLocals on reference types, merging two locals.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/support/small_vector.h b/src/support/small_vector.h
index 5791a0398..baf217086 100644
--- a/src/support/small_vector.h
+++ b/src/support/small_vector.h
@@ -111,6 +111,13 @@ public:
     flexible.clear();
   }
 
+  void resize(size_t newSize) {
+    usedFixed = std::min(N, newSize);
+    if (newSize > N) {
+      flexible.resize(newSize - N);
+    }
+  }
+
   bool operator==(const SmallVector<T, N>& other) const {
     if (usedFixed != other.usedFixed) {
       return false;