[MachineOutliner] NFC: Change IsTailCall to a call class + frame class

This commit

- Removes IsTailCall and replaces it with a target-defined unsigned
- Refactors getOutliningCallOverhead and getOutliningFrameOverhead so that they don't use IsTailCall
- Adds a call class + frame class classification to OutlinedFunction and Candidate respectively

This accomplishes a couple things.

Firstly, we don't need the notion of *tail call* in the general outlining algorithm.

Secondly, we now can have different "outlining classes" for each candidate within a set of candidates.
This will make it easy to add new ways to outline sequences for certain targets and dynamically choose
an appropriate cost model for a sequence depending on the context that that sequence lives in.

Ultimately, this should get us closer to being able to do something like, say avoid saving the link
register when outlining AArch64 instructions.

llvm-svn: 309475