[llvm] [CodeGen] Add a FunctionSplittingMode target option (PR #226575)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Add TargetOptions::FunctionSplitting and a -function-splitting={none,bbsections,all} flag. resolveFunctionSplittingMode() combines the flag with the legacy -split-machine-functions / -enable-split-machine-functions options and the basic block sections mode. MachineFunctionSplitter now runs when the resolved mode is 'all'.
> 

This should replace EnableMachineFunctionSplitter, not add a correlated flag 

https://github.com/llvm/llvm-project/pull/226575