[Liftoff] Fix emit_ptrsize_add

Use 64-bit addition if the pointer size is 8, use 32-bit addition
otherwise.

R=titzer@chromium.org
CC=​sreten.kovacevic@mips.com

Change-Id: I129cc29454726b7be45ecea13484212cc4f72eea
Reviewed-on: https://chromium-review.googlesource.com/998552Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52410}

src/wasm/baseline/liftoff-assembler.h

@@ -415,7 +415,7 @@ class LiftoffAssembler : public TurboAssembler {
                            Register amount, LiftoffRegList pinned = {});
 
   inline void emit_ptrsize_add(Register dst, Register lhs, Register rhs) {
-    if (kPointerSize == 4) {
+    if (kPointerSize == 8) {
       emit_i64_add(LiftoffRegister(dst), LiftoffRegister(lhs),
                    LiftoffRegister(rhs));
     } else {