[AArch64] Treat NOP as a separate instruction. (#170968)
Previously, nop was treated as just an alias for hint #0. The
consequence of that was that all the general rules for hint instructions
applied to nop too, in particular that during binary analysis, they were
assumed to have unknown effects. This commit adds AArch64::NOP as a
standalone instruction with no side effects.
The scheduling update in A55-load-store-alias.s is probably not entirely
accurate, but should be more accurate than the previous result.
