Teach x64 isel to eliminate some redundant zero extension #726

Job	Run time
Clippy (C API, Logging disabled)	31s
Clippy (C API, Logging enabled)	29s
Rust Tests	50s
C API Tests	57s
Rustfmt	13s
Check	34s
Clippy	42s
File Tests	54s
	5m 10s

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