Fix compressible string check for ctors with `hibyte`.

Only the low 8 bits of the `hibyte` are relevant but we
were checking the whole `hibyte`, erroneously creating
an uncompressed string when the low bits are zero but
high bits are non-zero. String.equals() depends on all
compressible Strings being actually compressed.

Test: Added regression test to 021-string2.
Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 31040547
Bug: 63661357
Change-Id: I31690b544de928ce9c6f39f3d144d3e2029f38f6
diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index 7560639..84587c8 100644
--- a/runtime/mirror/string-inl.h
+++ b/runtime/mirror/string-inl.h
@@ -251,6 +251,7 @@
                                           Handle<ByteArray> array, int32_t offset,
                                           int32_t high_byte, gc::AllocatorType allocator_type) {
   const uint8_t* const src = reinterpret_cast<uint8_t*>(array->GetData()) + offset;
+  high_byte &= 0xff;  // Extract the relevant bits before determining `compressible`.
   const bool compressible =
       kUseStringCompression && String::AllASCII<uint8_t>(src, byte_length) && (high_byte == 0);
   const int32_t length_with_flag = String::GetFlaggedCount(byte_length, compressible);
diff --git a/test/021-string2/src/Main.java b/test/021-string2/src/Main.java
index 194f4a1..3b81d8e 100644
--- a/test/021-string2/src/Main.java
+++ b/test/021-string2/src/Main.java
@@ -700,6 +700,11 @@
             $noinline$constNonAsciiString35Equals("\u0440123456789012345678901234567890123x"));
         Assert.assertFalse(
             $noinline$constNonAsciiString35Equals("01234567890123456789012345678901234"));
+
+        // Regression test for incorrectly creating an uncompressed string when the
+        // string should be compressed. Only the low 8 bits are relevant but the whole
+        // `hibyte` was erroneously tested. Bug: 63661357
+        Assert.assertTrue("A".equals(new String(new byte[] { (byte)'A' }, /* hibyte */ 0x100)));
     }
 
     public static boolean $noinline$equalsConstString0(String s) {