Avoid adding new unneeded names to blocks in text roundtripping (#4943)

Previously the wat parser would turn this input:

(block
  (nop)
)

into something like this:

(block $block17
  (nop)
)

It just added a name all the time, in case the block is referred to by an index
later even though it doesn't have a name.

This PR makes us rountrip more precisely by not adding such names: if there
was no name before, and there is no break by index, then do not add a name.

In addition, this will be useful for non-nullable locals since whether a block has
a name or not matters there. Like #4912, this makes us more regular in our
usage of block names.

1 files changed, 4 insertions, 2 deletions

diff --git a/test/lit/passes/avoid-reinterprets.wast b/test/lit/passes/avoid-reinterprets.wast
index 584016c97..046812dce 100644
--- a/test/lit/passes/avoid-reinterprets.wast
+++ b/test/lit/passes/avoid-reinterprets.wast
@@ -230,7 +230,7 @@
   ;; CHECK-NEXT:  )
   ;; CHECK-NEXT:  (drop
   ;; CHECK-NEXT:   (f32.reinterpret_i32
-  ;; CHECK-NEXT:    (block $block (result i32)
+  ;; CHECK-NEXT:    (block $a-name-avoids-fallthrough (result i32)
   ;; CHECK-NEXT:     (nop)
   ;; CHECK-NEXT:     (local.get $x)
   ;; CHECK-NEXT:    )
@@ -246,9 +246,11 @@
     )
     (drop
      (f32.reinterpret_i32
-      (block (result i32)
+      (block $a-name-avoids-fallthrough (result i32)
        (nop) ;; this would be removed by other opts, but in general, we can't
              ;; just look at the fallthrough, as we can't just remove code here
+             ;; (note that we need a name on the block, or else we would look at
+             ;; the fallthrough)
        (local.get $x)
       )
      )