LibJS/Bytecode: Add dedicated instruction for getting length property

By doing this, we can remove all the special-case checks for `length`
from the generic GetById and GetByIdWithThis code paths, making every
other property lookup a bit faster.

Small progressions on most benchmarks, and some larger progressions:

- 12% on Octane/crypto.js
- 6% on Kraken/ai-astar.js

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -57,6 +57,8 @@
     O(GetGlobal)                       \
     O(GetImportMeta)                   \
     O(GetIterator)                     \
+    O(GetLength)                       \
+    O(GetLengthWithThis)               \
     O(GetMethod)                       \
     O(GetNewTarget)                    \
     O(GetNextMethodFromIteratorRecord) \