JIT: reconsider use of BBF_DONT_REMOVE on EH blocks

[BruceForstall]

try/handler begin/last blocks are marked with BBF_DONT_REMOVE. Do we still need them to be? Can we check the block EH index instead when necessary to determine the EH status of a block? Can we generalize flow updates?

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Issue Details

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try/handler begin/last blocks are marked with BBF_DONT_REMOVE. Do we still need them to be? Can we check the block EH index instead when necessary to determine the EH status of a block? Can we generalize flow updates?

Author:	BruceForstall
Assignees:	BruceForstall
Labels:	area-CodeGen-coreclr
Milestone:	8.0.0