Optimize variable assignement

[bertysentry]

Problem

Most variable assignments require 3 opcodes: PUSH + ASSIGN + POP. See example below:

AWK script:

a = -2;
b = 2;
c = 4;

Which results in the below opcodes:

283 : PUSH_LONG, -2
284 : ASSIGN, 11, true
285 : POP

286 : PUSH_LONG, 2
287 : ASSIGN, 12, true
288 : POP

289 : PUSH_LONG, 4
290 : ASSIGN, 13, true
291 : POP

Specification

Most variable assignments don't need to be pushed on the stack, just to be poped immediately and discarded. Combinations of ASSIGN + POP should be simplified so that ASSIGN doesn't push to the stack.

In AVM.java, the below method should be updated to include a boolean push argument:

	/**
	 * Awk variable assignment functionality.
	 */
	private void assign(long l, Object value, boolean isGlobal, PositionTracker position, boolean push) {
		// check if curr value already refers to an array
		if (runtimeStack.getVariable(l, isGlobal) instanceof Map) {
			throw new AwkRuntimeException(position.lineNumber(), "cannot assign anything to an unindexed associative array");
		}
		if (push) { push(value); }
		runtimeStack.setVariable(l, value, isGlobal);
		// When specials are compiled correctly, they use ASSIGN_* and skip this path.
	}

And the optimization methods in AwkTuples.java must detect ASSIGN + POP opcodes and replace with a single ASSIGN(push: false) opcode (see notably peepholeOptimizePass()).