IR interpreter: Improve code gen for the main interpreter loop

Thanks to fp64 for the idea of using unreachable markers to avoid the
range check on the switch!

Additionally, use it in a few more places.

Common/CommonFuncs.h

@@ -75,6 +75,18 @@
 #include <x86intrin.h>
 #endif
 
+#ifdef _DEBUG
+#define UNREACHABLE() Crash()
+#else
+#if defined(_MSC_VER)
+#define UNREACHABLE() __assume(0)
+#elif defined(__GNUC__) || defined(__clang__)
+#define UNREACHABLE() __builtin_unreachable()
+#else
+#define UNREACHABLE() ((void)0)
+#endif
+#endif
+
 inline u32 __rotl(u32 x, int shift) {
 #if defined(_MSC_VER)
 	return _rotl(x, shift);

Common/GPU/Vulkan/VulkanQueueRunner.cpp

@@ -366,6 +366,9 @@ void VulkanQueueRunner::RunSteps(std::vector<VKRStep *> &steps, int curFrame, Fr
 			break;
 		case VKRStepType::RENDER_SKIP:
 			break;
+		default:
+			UNREACHABLE();
+			break;
 		}
 
 		if (profile && profile->timestampsEnabled && profile->timestampDescriptions.size() + 1 < MAX_TIMESTAMP_QUERIES) {
@@ -813,6 +816,7 @@ void VulkanQueueRunner::ApplyRenderPassMerge(std::vector<VKRStep *> &steps) {
 					break;
 				default:
 					// We added a new step?  Might be unsafe.
+					_dbg_assert_(false);
 					goto done_fb;
 				}
 			}
@@ -1239,7 +1243,7 @@ void VulkanQueueRunner::PerformRenderPass(const VKRStep &step, VkCommandBuffer c
 			break;
 
 		default:
-			ERROR_LOG(Log::G3D, "Unimpl queue command");
+			UNREACHABLE();
 			break;
 		}
 	}

Common/GPU/Vulkan/VulkanRenderManager.cpp

@@ -1637,6 +1637,7 @@ void VulkanRenderManager::Run(VKRRenderThreadTask &task) {
 
 	default:
 		_dbg_assert_(false);
+		break;
 	}
 
 	VLOG("PULL: Finished running frame %d", task.frame);
@@ -1739,7 +1740,7 @@ VKRPipelineLayout *VulkanRenderManager::CreatePipelineLayout(BindingType *bindin
 			bindings[i].stageFlags = VK_SHADER_STAGE_COMPUTE_BIT;
 			break;
 		default:
-			_dbg_assert_(false);
+			UNREACHABLE();
 			break;
 		}
 	}

Core/MIPS/IR/IRInterpreter.cpp

@@ -10,6 +10,7 @@
 #include "Common/BitSet.h"
 #include "Common/BitScan.h"
 #include "Common/Common.h"
+#include "Common/CommonFuncs.h"
 #include "Common/Data/Convert/SmallDataConvert.h"
 #include "Common/Math/math_util.h"
 #include "Common/Math/SIMDHeaders.h"
@@ -1257,9 +1258,9 @@ u32 IRInterpret(MIPSState *mips, const IRInst *inst) {
 		case IROp::Nop: // TODO: This shouldn't crash, but for now we should not emit nops, so...
 		case IROp::Bad:
 		default:
-			Crash();
+			// Unimplemented IR op. Bad. We define it as unreachable so the compiler can optimize better (remove the range check).
+			UNREACHABLE();
 			break;
-			// Unimplemented IR op. Bad.
 		}
 
 #ifdef _DEBUG