Fork reshape

Implements a fork-reshape pass in the scheduling language which can be used to coalesce a selection of fork-joins, interchange them into a specified order, and then split them into a specified series of chunks. Adds a parallel and tiled schedule for matmul that uses fork-reshape to keep the schedule quite clean.

Also adds tuples to the scheduling language and a new form of let-statements that allow destructing tuples, it's not the nicest (it's not a general pattern handler and will only cause an error if there are too many fields on the left-hand side, not too few, but it's enough to make the code that uses fork-reshape cleaner).

Includes a few other tweaks I ran into

• When adding a node in the editor, only insert an empty def_use set if the node doesn't have one yet (this is important due to cycles between nodes, and was causing issues in fork reshape since it invokes a series of passes using the same editor)
• Made fork-coalesce return the fork and join nodes when it succeeds so that they can be used in further operations.
• Modified fork coalesce's check for control flow between the forks and joins to check via uses rather than users because I was having weird issues with checking users
• Modified fork coalesce to perform everything as a single edit because I was having issues when manually scheduling matmul when using selections with coalesce.
• Modified fork split to return the ID of the new fork on a success
• Modified both fork split and coalesce to propagate parallel fork schedules when appropriate to avoid having to re-assign/re-infer schedules

juno_samples/matmul/src/gpu.sch

-phi-elim(*);
+macro optimize!(X) {
+  gvn(X);
+  phi-elim(X);
+  dce(X);
+  ip-sroa(X);
+  sroa(X);
+  dce(X);
+  gvn(X);
+  phi-elim(X);
+  dce(X);
+}
+
+macro codegen!(X) {
+  gcm(*);
+  float-collections(*);
+  dce(*);
+  gcm(*);
+}
 
-forkify(*);
-fork-guard-elim(*);
-dce(*);
+optimize!(*);
 
-fixpoint {
-  reduce-slf(*);
-  slf(*);
-  infer-schedules(*);
+fixpoint panic after 20 {
+  forkify(matmul);
+  fork-guard-elim(matmul);
 }
-fork-coalesce(*);
-infer-schedules(*);
-dce(*);
-rewrite(*);
-fixpoint {
-  simplify-cfg(*);
-  dce(*);
+
+optimize!(*);
+
+fixpoint panic after 20 {
+  reduce-slf(matmul);
+  slf(matmul);
+  infer-schedules(matmul);
 }
+dce(matmul);
 
-ip-sroa(*);
-sroa(*);
-dce(*);
+// Tile outer and middle loops into 32x32 sized blocks
+fork-tile[32, 0, false, true](matmul@outer \ matmul@inner);
+// Merge outer and middle loops and interchange so blocks are first
+fork-coalesce(matmul@outer \ matmul@inner);
+fork-interchange[1, 2](matmul@outer \ matmul@inner);
+// Split forks
+let split = fork-split(matmul);
+// Join the threads and then blocks into a single fork each
+fork-coalesce(split.matmul.fj2 \ matmul@inner);
+fork-coalesce(split.matmul.fj0 \ split.matmul.fj2);
 
+let auto = auto-outline(*);
 float-collections(*);
-gcm(*);
+gpu(auto.matmul);
+
+codegen!(*);