Clean up GenTreeCall's inline info

[EgorBo]

This PR slightly cleans up inline info for calls as a preparation for "multiple GDV candidates" (to reduce noise in future PRs for this):

1. Replaces field access with getters/setters (easy to debug what's set where)
2. Removes GuardedDevirtualizationCandidateInfo struct - it used to be a base "class" for InlineCandidateInfo and is smaller in size, so were always allocated as InlineCandidateInfo anyway.
3. Adds uint8_t gtInlineInfoCount; field to GenTreeCall - I've checked, it doesn't increase the struct size, I inserted the field where we had a 3 bytes padding.

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Issue Details

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This PR slightly cleans up inline info for calls as a preparation for "multiple GDV candidates":

1. Replaces field access with getters/setters (easy to debug what's set where)
2. Remove GuardedDevirtualizationCandidateInfo struct - it used to be a base "class" for InlineCandidateInfo and is smaller in size, so were always allocated as InlineCandidateInfo anyway.
3. uint8_t gtInlineInfoCount; field to GenTreeCall - I've checked, it doesn't increase the struct size, I inserted the field where we had a 3 bytes padding.

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

@AndyAyersMS PTAL, no-diffs change.

Next step will be NativeAOT case where we can use the 2nd GDV candidate as a fallback (if we know that we only have 2 classes)