[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.