Adds LdaGlobal bytecode and augments BytecodeGenerator to load globals for
global variables and function calls.

Modified TestBytecodeGenerator to add the ability to specify that a bytecode
operand has an unknown value (used so we don't need to figure out the slot
index of a global). Also added a helper which checks equality of BytecodeArray
with the expected snipptets.

Modified TestInterpreter to allow it to take snippets of JS and have the
BytecodeGenerator generate the bytecode rather than having to build a
BytecodeArray manually. This is used to enable the global tests.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1361113002

Cr-Commit-Position: refs/heads/master@{#30910}