[wasm-simd] Add overflowing value tests for splats

For I16x8Splat and I8x16Splat, the arguments takes I32, which can hold a
value larger than what should be splatted. We add tests to check that
the splatted values is the truncated I32 value (top bits masked off).

See WebAssembly/simd#151 for the updated to
proposal text.

Change-Id: Ib32770872e70c7cde2028130d2b44b416594610e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1986200
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65642}

test/cctest/wasm/test-run-wasm-simd.cc

@@ -1611,6 +1611,16 @@ WASM_SIMD_TEST(I16x8Splat) {
       CHECK_EQ(actual, expected);
     }
   }
+
+  // Test values that do not fit in a int16.
+  FOR_INT32_INPUTS(x) {
+    r.Call(x);
+    int16_t expected = truncate_to_int16(x);
+    for (int i = 0; i < 8; i++) {
+      int16_t actual = ReadLittleEndianValue<int16_t>(&g[i]);
+      CHECK_EQ(actual, expected);
+    }
+  }
 }
 
 WASM_SIMD_TEST(I16x8ReplaceLane) {
@@ -1660,6 +1670,16 @@ WASM_SIMD_TEST(I8x16Splat) {
       CHECK_EQ(actual, expected);
     }
   }
+
+  // Test values that do not fit in a int16.
+  FOR_INT16_INPUTS(x) {
+    r.Call(x);
+    int8_t expected = truncate_to_int8(x);
+    for (int i = 0; i < 16; i++) {
+      int8_t actual = ReadLittleEndianValue<int8_t>(&g[i]);
+      CHECK_EQ(actual, expected);
+    }
+  }
 }
 
 WASM_SIMD_TEST(I8x16ReplaceLane) {