Explicitly narrow Bool dests when appropriate in vasm

Summary: This ensures that the destination register for instructions like
t5:Bool = AssertType<Bool> t4:Cell will have the right type (i8) in llvm, and
it shouldn't affect the code generated by vasm-x64.

Reviewed By: @jasone

Differential Revision: D1746194

Signature: t1:1746194:1418849552:9c866c2c7ff77fc6333af6576687c96a13ebcc7f