general: Replace unreachable-imitating assertions with UNREACHABLE()

We can just use the self-documenting assertion for indicating
unreachable paths, instead of manually passing false and providing a
message.

src/backend/x64/a32_emit_x64.cpp

@@ -1018,7 +1018,7 @@ void A32EmitX64::EmitA32CoprocSendOneWord(A32EmitContext& ctx, IR::Inst* inst) {
         return;
     }
     default:
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 }
 
@@ -1060,7 +1060,7 @@ void A32EmitX64::EmitA32CoprocSendTwoWords(A32EmitContext& ctx, IR::Inst* inst)
         return;
     }
     default:
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 }
 
@@ -1102,7 +1102,7 @@ void A32EmitX64::EmitA32CoprocGetOneWord(A32EmitContext& ctx, IR::Inst* inst) {
         return;
     }
     default:
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 }
 
@@ -1147,7 +1147,7 @@ void A32EmitX64::EmitA32CoprocGetTwoWords(A32EmitContext& ctx, IR::Inst* inst) {
         return;
     }
     default:
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 }
 

src/backend/x64/emit_x64.cpp

@@ -128,7 +128,7 @@ void EmitX64::EmitGetNZCVFromOp(EmitContext& ctx, IR::Inst* inst) {
         case IR::Type::U64:
             return 64;
         default:
-            ASSERT_MSG(false, "Unreachable");
+            UNREACHABLE();
             return 0;
         }
     }();

src/backend/x64/oparg.h

@@ -26,7 +26,7 @@ struct OpArg {
         case Type::Reg:
             return inner_reg;
         }
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 
     void setBit(int bits) {
@@ -56,7 +56,7 @@ struct OpArg {
                 return;
             }
         }
-        ASSERT_MSG(false, "Unreachable");
+        UNREACHABLE();
     }
 
 private: