[llvm] [ValueTypes] Automatically assign value numbers (NFC) (PR #169071)

[Alex MacLean via llvm-commits]

================
@@ -11,11 +11,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-class ValueType<int size, int value> {
+class ValueType<int size, int value = -1> {
   string Namespace = "MVT";
   string LLVMName = NAME;
   int Size = size;
-  int Value = value;
+  int Value = !if(!ne(value, -1), value,
+                  !add(!size(!instances<ValueType>()), 1));
----------------
AlexMaclean wrote:

Yep. This is pretty inefficient, but there doesn't seem to be a better way to implement this in table-gen. 

I tested like this:

```
    git checkout main 
    echo "main"
    time ../cmake/bin/llvm-tblgen llvm/include/llvm/CodeGen/ValueTypes.td &> /dev/null

    git checkout dev/amaclean/td-vts-indices
    echo "dev/amaclean/td-vts-indices"
    time ../cmake/bin/llvm-tblgen llvm/include/llvm/CodeGen/ValueTypes.td &> /dev/null
```
Here are the results:
```
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
main

real    0m0.021s
user    0m0.018s
sys     0m0.002s
Switched to branch 'dev/amaclean/td-vts-indices'
dev/amaclean/td-vts-indices

real    0m0.094s
user    0m0.092s
sys     0m0.001s
```

So, when isolating ValueTypes.td, it makes things nearly 5X slower (which is pretty bad) but we're still only talking about less then 1/10th of a second which seems like it will be pretty negligible on the scale of a complete LLVM build. 

https://github.com/llvm/llvm-project/pull/169071