Allow comparing codegen backends on the compare page

[Kobzol]

Currently, the compare page is mostly designed for comparing two different compiler artifacts. However, in some cases, it would be very useful to compare different benchmark configurations of the same compiler artifact. The most useful example are codegen backends.

We should add some support for this to the compare page. If we are comparing the same artifact against itself, we could add some select box that would allow the user to select which parameter they want to use for the "self-comparison", and then select the before/after values that will be used for the comparison (e.g. Cranelift vs LLVM).