IR interpreter: Replace the assert for downcount with a very predictable branch #9963

Job	Run time
build-uwp	8m 45s
build-windows (x64)	10m 7s
build (ubuntu-latest, test, clang, clang++, ./b.sh --headless --unittest, clang-normal)	3m 33s
build-windows (ARM64)	9m 50s
build_test_headless_alpine	12m 52s
build (ubuntu-latest, gcc, g++, ./b.sh --headless --unittest, gcc-normal)	3m 11s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=arm64-v8a UNITTE...	9m 25s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=armeabi-v7a UNIT...	6m 23s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=x86_64 UNITTEST=...	10m 12s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=arm64-v8a OPENXR...	8m 54s
build (ubuntu-latest, android, clang, clang++, ./b.sh --libretro_android ppsspp_libretro, android...	4m 38s
build (ubuntu-latest, qt, gcc, g++, ./b.sh --qt, qt)	3m 24s
build (ubuntu-latest, libretro, gcc, g++, make -C libretro -f Makefile -j2, gcc-libretro)	2m 21s
build (ubuntu-latest, libretro, clang, clang++, make -C libretro -f Makefile -j2, clang-libretro)	2m 31s
build (macos-latest, test, clang, clang++, ./b.sh --headless --unittest --fat --no-png --no-sdl2,...	9m 4s
build (macos-latest, ios, clang, clang++, ./b.sh --ios, ios)	3m 32s
test-windows	53s
test (ubuntu-latest)	53s
test (macos-latest)	40s
	1h 51m 8s

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