irgen: JVM load instructions are (sometimes) handled incorrectly

[quasilyte]

Our algorithm works in the simplest cases, but it does fail badly in cases where we have gotos (see #37).

We could use the approach outlined in Virtual Machine Showdown: Stack Versus Registers, but it would require doing more optimizations for the generated IR:

1. More unoptimized IR nodes allocations (that we want to remove later).
2. It would need more time to make code optimized afterward.
3. Less optimized code with -opt=0 level.

The current schema, of course, does not generate valid code.
If we can fix it without the above-mentioned drawbacks, that would be an optimal solution.