- Added a hook processFunctionBeforeCalleeSaveScn(). This is called by PEI just

- Added a hook processFunctionBeforeCalleeSaveScn(). This is called by PEI just
before it determines which callee-save registers are to be spilled. This allows
the target to make changes such as forcing certain physical registers to be
spilled.
- Modified comments. It's important to note the order of registers in the array
returns by getCalleeSaveRegs() determines the order of callee save spill code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30635 91177308-0d34-0410-b5e6-96231b3b80d8