ReFinalize in MergeBlocks so we can optimize unreachable instructions too (#6994)

In #6984 we optimized dropped blocks even if they had unreachable code. In #6988
that part was reverted, and blocks with unreachable code were ignored once more.
However, I realized that the check was not actually for unreachable code, but for
having an unreachable child, so it would miss things like this:

(block
  (block
    ..
    (br $somewhere) ;; unreachable type, but no unreachable code
  )
)

But it is useful to merge such blocks: we don't need the inner block here.

To fix this, just run ReFinalize if we change anything, which will propagate
unreachability as needed. I think MergeBlocks was written before we had
that utility, so it didn't use it...

This is not only useful for itself but will unblock an EH optimization in a
later PR, that has code in this form. It also simplifies the code by removing
the hasUnreachableChild checks.

1 files changed, 4 insertions, 5 deletions

diff --git a/test/lit/passes/merge-blocks.wast b/test/lit/passes/merge-blocks.wast
index 6fa1687b8..86181f7a4 100644
--- a/test/lit/passes/merge-blocks.wast
+++ b/test/lit/passes/merge-blocks.wast
@@ -405,12 +405,11 @@
  )
 
  ;; CHECK:      (func $toplevel (type $4)
- ;; CHECK-NEXT:  (drop
- ;; CHECK-NEXT:   (block $label (result i32)
- ;; CHECK-NEXT:    (br $label
- ;; CHECK-NEXT:     (i32.const 42)
- ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:  (block $label
+ ;; CHECK-NEXT:   (drop
+ ;; CHECK-NEXT:    (i32.const 42)
  ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (br $label)
  ;; CHECK-NEXT:  )
  ;; CHECK-NEXT: )
  (func $toplevel