Missing peephole optimizations in parser, compiler

There appears to be a number of peephole optimizations we could add that may not add up to a lot of improvement, but could be useful under some circumstances. For example, a few I know of (Marcin may know more):

• Bare literals on a line are not executed
• Repeated retrievals of the same local variable with no intervening code are reduced to a single retrieval or not executed at all
• Repeated retrievals of an instance variable with no intervening code are reduced to a single retrieval

All of these tend to confound benchmarking, but they may also be easy for us to add to reduce (slightly) overhead in cases of badly-written code.