PowerPC: Fix register spilling for SPE registers

Summary:
Missed in the original commit, use the correct callee-saved register
list for spilling, instead of the standard SVR432 list.  This avoids
needlessly spilling the SPE non-volatile registers when they're not used.

As part of this, also add where missing, and sort, the spill opcode
checks for SPE and SPE4 register classes.

Reviewers: nemanjai, hfinkel, joerg

Subscribers: kbarton, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D56703

llvm-svn: 366319