Require `then` and `else` with `if` (#6201)

We previously supported (and primarily used) a non-standard text format for
conditionals in which the condition, if-true expression, and if-false expression
were all simply s-expression children of the `if` expression. The standard text
format, however, requires the use of `then` and `else` forms to introduce the
if-true and if-false arms of the conditional. Update the legacy text parser to
require the standard format and update all tests to match. Update the printer to
print the standard format as well.

The .wast and .wat test inputs were mechanically updated with this script:
https://gist.github.com/tlively/85ae7f01f92f772241ec994c840ccbb1

4 files changed, 44 insertions, 22 deletions

diff --git a/test/lld/basic_safe_stack.wat.out b/test/lld/basic_safe_stack.wat.out
index 617463a66..7d554cc88 100644
--- a/test/lld/basic_safe_stack.wat.out
+++ b/test/lld/basic_safe_stack.wat.out
@@ -32,8 +32,10 @@
      (global.get $__stack_limit)
     )
    )
-   (call $__handle_stack_overflow
-    (local.get $1)
+   (then
+    (call $__handle_stack_overflow
+     (local.get $1)
+    )
    )
   )
   (global.set $__stack_pointer
@@ -66,8 +68,10 @@
       (global.get $__stack_limit)
      )
     )
-    (call $__handle_stack_overflow
-     (local.get $3)
+    (then
+     (call $__handle_stack_overflow
+      (local.get $3)
+     )
     )
    )
    (global.set $__stack_pointer
diff --git a/test/lld/em_asm_pthread.wasm.out b/test/lld/em_asm_pthread.wasm.out
index 40f61e3a7..3b51b896a 100644
--- a/test/lld/em_asm_pthread.wasm.out
+++ b/test/lld/em_asm_pthread.wasm.out
@@ -134,14 +134,16 @@
     (i32.const 0)
     (i32.const 1)
    )
-   (drop
-    (memory.atomic.wait32
-     (i32.const 4032)
-     (i32.const 1)
-     (i64.const -1)
+   (then
+    (drop
+     (memory.atomic.wait32
+      (i32.const 4032)
+      (i32.const 1)
+      (i64.const -1)
+     )
     )
    )
-   (block
+   (else
     (memory.init $0
      (i32.const 1024)
      (i32.const 0)
diff --git a/test/lld/recursive_safe_stack.wat.out b/test/lld/recursive_safe_stack.wat.out
index 32c79c310..ad30dc776 100644
--- a/test/lld/recursive_safe_stack.wat.out
+++ b/test/lld/recursive_safe_stack.wat.out
@@ -46,8 +46,10 @@
       (global.get $__stack_limit)
      )
     )
-    (call $__handle_stack_overflow
-     (local.get $3)
+    (then
+     (call $__handle_stack_overflow
+      (local.get $3)
+     )
     )
    )
    (global.set $global$0
@@ -85,8 +87,10 @@
       (global.get $__stack_limit)
      )
     )
-    (call $__handle_stack_overflow
-     (local.get $4)
+    (then
+     (call $__handle_stack_overflow
+      (local.get $4)
+     )
     )
    )
    (global.set $global$0
@@ -121,8 +125,10 @@
       (global.get $__stack_limit)
      )
     )
-    (call $__handle_stack_overflow
-     (local.get $1)
+    (then
+     (call $__handle_stack_overflow
+      (local.get $1)
+     )
     )
    )
    (global.set $global$0
@@ -159,8 +165,10 @@
       (global.get $__stack_limit)
      )
     )
-    (call $__handle_stack_overflow
-     (local.get $2)
+    (then
+     (call $__handle_stack_overflow
+      (local.get $2)
+     )
     )
    )
    (global.set $global$0
diff --git a/test/lld/safe_stack_standalone-wasm.wat.out b/test/lld/safe_stack_standalone-wasm.wat.out
index 353e44b3a..63d6c09d4 100644
--- a/test/lld/safe_stack_standalone-wasm.wat.out
+++ b/test/lld/safe_stack_standalone-wasm.wat.out
@@ -44,7 +44,9 @@
       (global.get $__stack_limit)
      )
     )
-    (unreachable)
+    (then
+     (unreachable)
+    )
    )
    (global.set $global$0
     (local.get $3)
@@ -81,7 +83,9 @@
       (global.get $__stack_limit)
      )
     )
-    (unreachable)
+    (then
+     (unreachable)
+    )
    )
    (global.set $global$0
     (local.get $4)
@@ -115,7 +119,9 @@
       (global.get $__stack_limit)
      )
     )
-    (unreachable)
+    (then
+     (unreachable)
+    )
    )
    (global.set $global$0
     (local.get $1)
@@ -151,7 +157,9 @@
       (global.get $__stack_limit)
      )
     )
-    (unreachable)
+    (then
+     (unreachable)
+    )
    )
    (global.set $global$0
     (local.get $2)