Fix five ways to end up stuck, crashed, or silently degraded

System.cpp stamped BootState::Complete unconditionally after InitGPU(), overwriting
the Failed that InitGPU sets when GPU_Init() fails - after it has already run
CPU_Shutdown(). PSP_InitUpdate then took the success path on a core that no longer
existed, down to a null Memory::base, and the first guest access dereferenced it.
InitGPU now reports failure and both callers honor it. (The libretro path had the
same problem from the other direction: it calls InitGPU after the Failed check.)

HandleAssert called g_assertCancelCallback directly on the IDCANCEL path, without
the null check its own BreakIntoPSPDebugger() helper does - and EmuScreen clears the
callback when a game is unloaded. So any assert after returning to the menu turned
"Cancel: skip and break into PPSSPP debugger" into a null jump, from the one button
whose entire purpose is surviving the assert.

__CheatDoState registered the cheat event type when the savestate had no CwCheat
section, but never scheduled it. CoreTiming::DoState has already swapped in the
state's event queue by then, which doesn't contain one either - so loading an old
savestate silently killed cheats, and the enable/disable polling with them, for the
rest of the session.

Achievements::ChangeUMD set g_isIdentifying and returned without clearing it when
hashing failed, leaving IsBlockingExecution() true forever - EmuScreen stops running
the CPU and the game is frozen until restart. Reachable from a disc swap on any ISO
whose PARAM.SFO or EBOOT.BIN can't be read.

x64Analyzer routed opcode 0x88 into the write path but had no case for it, so it hit
the default, logged from inside the crash handler, and failed. 0x88 is exactly what
the x64 JIT emits for a guest sb, so MemFault could never skip or ignore a bad byte
store the way it can a word one. Handle the 8-bit forms, and drop the 0x8a/0x8b cases
in the read path that the same 0xF0 mask made unreachable. Covered by a new
CheckAnalyze case, which fails without this change.

314 pspautotests pass, all unit tests pass.

Core/System.cpp

@@ -664,7 +664,9 @@ void PSP_ForceDebugStats(bool enable) {
 	_assert_(g_coreCollectDebugStatsCounter >= 0);
 }
 
-static void InitGPU(std::string *error_string) {
+// Returns false if the GPU couldn't be brought up - in which case it has already set
+// BootState::Failed and torn the CPU back down, so the caller must not carry on.
+static bool InitGPU(std::string *error_string) {
 	if (!gpu) {  // should be!
 		INFO_LOG(Log::Loader, "Starting graphics...");
 		Draw::DrawContext *draw = g_CoreParameter.graphicsContext ? g_CoreParameter.graphicsContext->GetDrawContext() : nullptr;
@@ -675,8 +677,10 @@ static void InitGPU(std::string *error_string) {
 			*error_string = "Unable to initialize rendering engine.";
 			CPU_Shutdown(false);
 			g_bootState = BootState::Failed;
+			return false;
 		}
 	}
+	return true;
 }
 
 bool PSP_InitStart(const CoreParameter &coreParam) {
@@ -752,7 +756,12 @@ bool PSP_InitStart(const CoreParameter &coreParam) {
 		// Initialize the GPU as far as we can here (do things like load cache files).
 		_dbg_assert_(!gpu);
 #ifndef __LIBRETRO__
-		InitGPU(errorString);
+		// Must not stamp Complete over the Failed that InitGPU sets - it has already run
+		// CPU_Shutdown(), so PSP_InitUpdate would take the success path on a core that no longer
+		// exists, right down to a null Memory::base.
+		if (!InitGPU(errorString)) {
+			return;
+		}
 #endif
 		g_bootState = BootState::Complete;
 	});
@@ -784,7 +793,13 @@ BootState PSP_InitUpdate(std::string *error_string) {
 	}
 
 #ifdef __LIBRETRO__
-	InitGPU(error_string);
+	if (!InitGPU(error_string)) {
+		// Same as the Failed branch above - the core is already gone.
+		Core_NotifyLifecycle(CoreLifecycle::START_COMPLETE);
+		*error_string = g_CoreParameter.errorString;
+		g_bootState = BootState::Off;
+		return BootState::Failed;
+	}
 #endif
 
 	// Ok, async part of the boot completed, let's finish up things on the main thread.