[Liftoff][arm64] Fix i32.popcnt

Only use the "W" part (lower 32 bit) of the src register. Otherwise, we
can get results larger than 32.

R=ahaas@chromium.org
CC=​rodolph.perfetta@arm.com

Bug: v8:7914, chromium:854011
Change-Id: I6329231e6cc0ae537c165b2d383fc5a14bd28ca3
Reviewed-on: https://chromium-review.googlesource.com/1122409
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54152}

src/wasm/baseline/arm64/liftoff-assembler-arm64.h

@@ -478,7 +478,7 @@ bool LiftoffAssembler::emit_i32_ctz(Register dst, Register src) {
 bool LiftoffAssembler::emit_i32_popcnt(Register dst, Register src) {
   UseScratchRegisterScope temps(this);
   VRegister scratch = temps.AcquireV(kFormat8B);
-  Fmov(scratch, src.X());
+  Fmov(scratch.S(), src.W());
   Cnt(scratch, scratch);
   Addv(scratch.B(), scratch);
   Fmov(dst.W(), scratch.S());