PPC: Allocate RS spill slot for unaligned i64 load/store

This fixes another bug found by llvm-stress!

If we happen to be doing an i64 load or store into a stack slot that has less
than a 4-byte alignment, then the frame-index elimination may need to use an
indexed load or store instruction (because the offset may not be a multiple of
4, a requirement of the STD/LD instructions). The extra register needed to hold
the offset comes from the register scavenger, and it is possible that the
scavenger will need to use an emergency spill slot. As a result, we need to
make sure that a spill slot is allocated when doing an i64 load/store into a
less-than-4-byte-aligned stack slot.

Because test cases for things like this tend to be fairly fragile, I've
concatenated a few small bugpoint-reduced test cases together to form the
regression test.