Bug 1574865 - Wasm: Update encoding of nullref to match proposal. r=lth

This commit updates the encoding of nullref to match the one given in
the reference types proposal.

Spec: WebAssembly/reference-types#66

Differential Revision: https://phabricator.services.mozilla.com/D58882

UltraBlame original commit: 985cd0a6aca99bb5bb0b0ad8d2225a019bdf1d58

Author: marco-c

js/src/jit-test/lib/wasm-binary.js

@@ -246,7 +246,7 @@ AnyrefCode
 const
 RefCode
 =
-0x6e
+0x6d
 ;
 const
 FuncCode

js/src/wasm/WasmConstants.h

@@ -420,6 +420,29 @@ AnyRef
 0x6f
 /
 /
+SLEB128
+(
+-
+0x11
+)
+/
+/
+A
+null
+reference
+.
+NullRef
+=
+0x6e
+/
+/
+SLEB128
+(
+-
+0x12
+)
+/
+/
 Type
 constructor
 for
@@ -428,7 +451,7 @@ types
 .
 Ref
 =
-0x6e
+0x6d
 /
 /
 Type
@@ -485,24 +508,6 @@ SLEB128
 -
 0x40
 )
-/
-/
-Type
-designator
-for
-null
--
-unofficial
-will
-not
-appear
-in
-the
-binary
-format
-NullRef
-=
-0x39
 Limit
 =
 0x80