This is directly available in TargetInstrInfo

This patch adds the MIR parsing and serialization support for save and
restore points with subsets of callee saved registers. That is, it
syntactically allows a function to contain two or more distinct
sub-regions in which distinct subsets of registers are spilled/filled as
callee save. This is useful if e.g. one of the CSRs isn't modified in
one of the sub-regions, but is in the other(s).

Support for actually using this capability in code generation is still
forthcoming. This patch is the next logical step for multiple
save/restore points support.

All points are now stored in DenseMap from MBB to vector of
CalleeSavedInfo.

Shrink-Wrap points split Part 4.
RFC:
https://discourse.llvm.org/t/shrink-wrap-save-restore-points-splitting/83581

Part 1: https://github.com/llvm/llvm-project/pull/117862 (landed)
Part 2: https://github.com/llvm/llvm-project/pull/119355 (landed)
Part 3: https://github.com/llvm/llvm-project/pull/119357 (landed)
Part 5: https://github.com/llvm/llvm-project/pull/119359 (likely to be
further split)

Currently mir supports only one save and one restore point
specification:

```
  savePoint:       '%bb.1'
  restorePoint:    '%bb.2'
```

This patch provide possibility to have multiple save and multiple
restore points in mir:

```
  savePoints:
    - point:           '%bb.1'
  restorePoints:
    - point:           '%bb.2'
```

Shrink-Wrap points split Part 3.
RFC:
https://discourse.llvm.org/t/shrink-wrap-save-restore-points-splitting/83581

Part 1: https://github.com/llvm/llvm-project/pull/117862
Part 2: https://github.com/llvm/llvm-project/pull/119355
Part 4: https://github.com/llvm/llvm-project/pull/119358
Part 5: https://github.com/llvm/llvm-project/pull/119359

Co-authored-by: Nikita Popov <github@npopov.com>

Callee saved registers should always be phyiscal registers. They are
often passed directly to other functions that take MCRegister like
getMinimalPhysRegClass or TargetRegisterClass::contains.

Unfortunately, sometimes the MCRegister is compared to a Register which
gave an ambiguous comparison error when the MCRegister is on the LHS.
Adding a MCRegister==Register comparison operator created more ambiguous
comparison errors elsewhere. These cases were usually comparing against
a base or frame pointer register that is a physical register in a
Register. For those I added an explicit conversion of Register to
MCRegister to fix the error.

An optimization was added that tries to move the uses of the mflr
instruction away from the instruction itself. However, this doesn't work
when we are using the hashst instruction because that instruction needs
to be run before the stack frame is obtained.

This patch disables moving instructions away from the mflr in the case
where ROP protection is being used.

---------

Co-authored-by: Lei Huang <lei@ca.ibm.com>