Add support for manual common subexpression elimination

[charleskawczynski]

It seems that MultiBroadcastFusion is not fusing reads/writes in ClimaCore. Perhaps the compiler is unable to perform the optimization (these are pretty complicated expressions afterall). If the compiler is unable to fuse the reads and writes in a multi-broadcast fusion object, then we could try to write a function that manually does this by transforming the object.

Some important examples might include:

(Pair(x, Broadcasted(+, x, 1)),
Pair(x, Broadcasted(-, x, 2)),
Pair(x, Broadcasted(*, x, 3)),
Pair(x, Broadcasted(/, x, 4)))

would be transformed to

(Pair(x, Broadcasted(/, Broadcasted(*, Broadcasted(-, Broadcasted(+, x, 1), 2), 3), 4)),)

and

(Pair(z, Broadcasted(+, a, b)),
Pair(x, Broadcasted(-, z, a)),
Pair(y, Broadcasted(*, z, b)))

transformed to

?

Hm, I'm not sure how to hoist here without things inlined.. All of the reads/writes exist inside a single getindex call, but if hoisted reads/writes depend on all the reads to exist inside a single _broadcast_getindex call, then we'll need to transform the MBF object into a custom broadcasted object and have it return a tuple, like @dennisYatunin suggested.

Either way, this sort of transformation should guarantee avoiding duplicate reads/writes in the first case, and (hopefully) lead to fused reads/writes in the latter case.

[charleskawczynski]

xref: https://github.com/rdeits/CommonSubexpressions.jl, https://github.com/JuliaLang/julia/blob/30a73de68108b9629c4c99a1751d579ef5893bda/base/compiler/ssair/passes.jl#L1715