Throw NPE (instead of UnsupportedOperationException) if an invalid en…

…um value is detected.

This should never happen, so I don't think it matters much exactly what kind of
exception we throw. We could even arguably return null, but this option saves a
lot of space while still preserving some error checking.

See https://godbolt.org/z/jKhcKs3x1 for code gen.

This generates much tighter bytecode and ARM assembly than alternatives.

As this code is generated many times over, small wins in code size here can
reduce icache pressure, APK size, and OAT size.

This java code:

```java
  Object uoe() {
       throw new UnsupportedOperationException();
  }
  Object npe2() {
    throw null;
  }
```

Generates this dex code:

```
.method uoe()Ljava/lang/Object;
    new-instance v0, Ljava/lang/UnsupportedOperationException;
    invoke-direct {v0}, Ljava/lang/UnsupportedOperationException;-><init>()V
    throw v0
.end method

.method npe2()Ljava/lang/Object;
    const/4 v0, 0x0
    throw v0
.end method
```

Which generates this OAT code:

```
java.lang.Object SomeProto.uoe() [84 bytes]
    0x000081c0    sub x16, sp, #0x2000 (8192)
    0x000081c4    ldr wzr, [x16]
     StackMap[0]   native_pc=0x41c8, dex_pc=0x0, register_mask=0x0, stack_mask=0b
    0x000081c8    str x0, [sp, #-48]!
    0x000081cc    str x22, [sp, #24]
    0x000081d0    stp x23, lr, [sp, #32]
    0x000081d4    ldr x21, [x21]
     StackMap[1]   native_pc=0x41d8, dex_pc=0x0, register_mask=0x2, stack_mask=0b
    0x000081d8    mov x22, x1
    0x000081dc    adrp x0, #+0x4000 (addr 0x0000c000)
    0x000081e0    ldr w0, [x0, #4]
    0x000081e4    ldr lr, [tr, #464] ; pAllocObjectInitialized
    0x000081e8    blr lr
     StackMap[2]   native_pc=0x41ec, dex_pc=0x0, register_mask=0x400000, stack_mask=0b
    0x000081ec    dmb ishst
    0x000081f0    mov x1, x0
    0x000081f4    mov x23, x1
    0x000081f8    adrp x0, #+0x4000 (addr 0x0000c000)
    0x000081fc    ldr w0, [x0, #12]
    0x00008200    ldr lr, [x0, #24]
    0x00008204    blr lr
     StackMap[3]   native_pc=0x4208, dex_pc=0x2, register_mask=0xc00000, stack_mask=0b
    0x00008208    mov x0, x23
    0x0000820c    ldr lr, [tr, #1264] ; pDeliverException
    0x00008210    blr lr
     StackMap[4]   native_pc=0x4214, dex_pc=0x5, register_mask=0xc00000, stack_mask=0b

java.lang.Object SomeProto.npe2() [36 bytes]
    0x000080d0    sub x16, sp, #0x2000 (8192)
    0x000080d4    ldr wzr, [x16]
     StackMap[0]   native_pc=0x40d8, dex_pc=0x0, register_mask=0x0, stack_mask=0b
    0x000080d8    str x0, [sp, #-32]!
    0x000080dc    stp x22, lr, [sp, #16]
    0x000080e0    ldr x21, [x21]
     StackMap[1]   native_pc=0x40e4, dex_pc=0x0, register_mask=0x2, stack_mask=0b
    0x000080e4    mov x22, x1
    0x000080e8    mov w0, #0x0
    0x000080ec    ldr lr, [tr, #1264] ; pDeliverException
    0x000080f0    blr lr
     StackMap[2]   native_pc=0x40f4, dex_pc=0x1, register_mask=0x400000, stack_mask=0b
```

This saves 84-36 = 48 bytes of OAT per method.

PiperOrigin-RevId: 684258075

src/google/protobuf/compiler/java/lite/message.cc

@@ -383,8 +383,6 @@ void ImmutableMessageLiteGenerator::Generate(io::Printer* printer) {
       "  return parser;\n",
       "classname", name_resolver_->GetImmutableClassName(descriptor_));
 
-  printer->Outdent();
-
   if (HasRequiredFields(descriptor_)) {
     printer->Print(
         "}\n"
@@ -408,11 +406,13 @@ void ImmutableMessageLiteGenerator::Generate(io::Printer* printer) {
 
   printer->Outdent();
   printer->Print(
-      "  }\n"
-      "  throw new UnsupportedOperationException();\n"
       "}\n"
-      "\n",
-      "classname", name_resolver_->GetImmutableClassName(descriptor_));
+      "// Should never happen. Generates tight code to throw an exception.\n"
+      "throw null;\n");
+  printer->Outdent();
+  printer->Print(
+      "}\n"
+      "\n");
 
   printer->Print(
       "\n"