[PPC] Fix two bugs in frame lowering.

1. The available program storage region of the red zone to compilers is 288
 bytes rather than 244 bytes.
2. The formula for negative number alignment calculation should be
y = x & ~(n-1) rather than y = (x + (n-1)) & ~(n-1).

Differential Revision: https://reviews.llvm.org/D34337

llvm-svn: 307672