arm64/nterp: Move opcode dispatch to some handlers.

Since we shortened the opcode dispatch by one instruction,
we can move the dispatch back to the main handlers in some
cases.

Also move the read barrier to the main handler for some
`sput*` opcodes.

Test: testrunner.py --target --64 --interpreter
Test: Repeat with `ART_HEAP_POISONING=true`
Flag: EXEMPT PURE_REFACTOR
Change-Id: Id96e3c0362ee90f2f78057d162916727bf429c0e