[PowerPC] Don't spill the frame pointer twice

When a function contains something, such as inline asm, which explicitly
clobbers the register used as the frame pointer, don't spill it twice. If we
need a frame pointer, it will be saved/restored in the prologue/epilogue code.
Explicitly spilling it again will reuse the same spill slot used by the
prologue/epilogue code, thus clobbering the saved value. The same applies
to the base-pointer or PIC-base register.

Partially fixes PR26856. Thanks to Ulrich for his analysis and the small
inline-asm reproducer.

llvm-svn: 280188