JIT: Modify switch and finally-ret successors using edge redirection helpers

[amanasifkhalid]

Follow-up to #116933. By mutating the existing edge during switch successor modifications, we can avoid invalidating the unique switch successor map. Also adds a helper for removing edges from the switch successor map without needing to trash the whole entry. I'd like to get to a point where we always have accurate switch successor sets available -- we could even consider rearchitecting BBswtDesc to model BBehfDesc and hold unique successor edges.

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[Copilot]

Pull Request Overview

This PR refines switch successor updates by mutating existing edges and adding helpers to remove specific switch desc map entries, avoiding full invalidations. Key changes include:

• Replace full map invalidation with targeted edge removal in LowerSwitch.
• Introduce GetDescriptorForSwitchIfAvailable and fgRemoveSuccFromSwitchDescMapEntry to manage switch successor sets.
• Refactor fgReplaceEhfSuccessor to use fgRedirectEdge and a renamed removal helper for cleaner successor table updates.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/lower.cpp	Use fgRemoveSuccFromSwitchDescMapEntry when peeling off the default case and invalidate only when needed.
src/coreclr/jit/fgflow.cpp	Add GetDescriptorForSwitchIfAvailable and fgRemoveSuccFromSwitchDescMapEntry; deprecate global invalidation.
src/coreclr/jit/fgbasic.cpp	Refactor fgReplaceEhfSuccessor to use edge redirection, rename fgRemoveEhfSuccessor, and optimize duplicate handling.
src/coreclr/jit/compiler.h	Remove InvalidateUniqueSwitchSuccMap, declare new helpers, rename fgRemoveEhfSuccessor.

[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Small diffs from floating-point precision changes around edge weights. Thanks!