The previous attempt, which made do with a single offset in computeCalleeSaveRegisterPairs, wasn't quite enough. The previous attempt only worked as long as CombineSPBump == true (since the offset would be adjusted later in fixupCalleeSaveRestoreStackOffset).
Instead include the size for the fixed stack area used for win64 varargs in calculations in emitPrologue/emitEpilogue. The stack consists of mainly three parts;
Most of the places in the code which previously used the CSStackSize now use PrologueSaveSize instead, which is the sum of the latter two, while some cases which need exactly the middle one use AFI->getCalleeSavedStackSize() explicitly instead of a local variable.
In addition to moving the offsetting into emitPrologue/emitEpilogue (which fixes functions with CombineSPBump == false), also set the frame pointer to point to the right location, where the frame pointer and link register actually are stored. In addition to the prologue/epilogue, this also requires changes to resolveFrameIndexReference.
Add tests for a function that keeps a frame pointer and that uses a VLA.