[liftoff][wasm-simd] Implement i16x8.splat on arm and arm64

This is the same as codegen used in TurboFan, with the registers being
referred to in a different way.

Bug: v8:9909
Change-Id: I082caabc49d6adb367b103a77e44a4e6e1be0ac1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2060265Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66324}

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

@@ -1550,7 +1550,7 @@ void LiftoffAssembler::emit_i32x4_splat(LiftoffRegister dst,
 
 void LiftoffAssembler::emit_i16x8_splat(LiftoffRegister dst,
                                         LiftoffRegister src) {
-  bailout(kSimd, "i16x8splat");
+  vdup(Neon16, liftoff::GetSimd128Register(dst.low_fp()), src.gp());
 }
 
 void LiftoffAssembler::StackCheck(Label* ool_code, Register limit_address) {

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

@@ -1081,7 +1081,7 @@ void LiftoffAssembler::emit_i32x4_splat(LiftoffRegister dst,
 
 void LiftoffAssembler::emit_i16x8_splat(LiftoffRegister dst,
                                         LiftoffRegister src) {
-  bailout(kSimd, "i16x8splat");
+  Dup(dst.fp().V8H(), src.gp().W());
 }
 
 void LiftoffAssembler::StackCheck(Label* ool_code, Register limit_address) {