JIT: Add helpers for increasing/decreasing block weights

[amanasifkhalid]

In light of the profile consistency work part of #107749, this seems like a useful abstraction.

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[amanasifkhalid]

I was expecting this to be a no-diff change, though there is one diff on linux-arm in libraries_tests.run. The diff is from the 4-opt component of block layout choosing a different cut point:

Initial layout cost: 421.913383
Running greedy 3-opt pass.
Swapping partitions [BB179, BB161] and [BB162, BB298] (cost change = -32.000000)
- Swapping partitions [BB162, BB296] and [BB180, BB160] (cost change = -2.000000)
+ Swapping partitions [BB162, BB191] and [BB186, BB160] (cost change = -2.000000)
Final layout cost: 387.913383

The comparison of two seemingly-equal layout costs flips with this change; I'm guessing there's some subtle floating-point imprecision at play. Since the JIT still converged to the same layout cost, I think this is fine.

[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Thanks!