Prevent despeculation of count-zero operations on Z80 #30

Job	Run time
build-llvm (macos-14, Release)	20m 35s
build-llvm (ubuntu-22.04, Release)	37m 57s
build-llvm (macos-13, Release)	44m 16s
build-llvm (ubuntu-latest, Release)	40m 27s
test-llvm-z80 (ubuntu-22.04, Release)	1m 30s
test-llvm-z80 (ubuntu-latest, Release)	1m 19s
build-clang (macos-13, Release)	34m 52s
build-clang (macos-14, Release)	14m 28s
test-llvm-z80 (macos-13, Release)	2m 13s
build-clang (ubuntu-22.04, Release)	36m 38s
test-llvm-z80 (macos-14, Release)	1m 26s
build-clang (ubuntu-latest, Release)	37m 44s
test-llvm	0s
nightly	0s
test-clang	0s
	4h 33m 25s

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