Fix a problem that Nate noticed, where spill code was not getting coallesced

with copies, leading to code like this:

       lwz r4, 380(r1)
       or r10, r4, r4    ;; Last use of r4

By teaching the PPC backend how to fold spills into copies, we now get this
code:

       lwz r10, 380(r1)

wow. :)

This reduces a testcase nate sent me from 1505 instructions to 1484.

Note that this could handle FP values but doesn't currently, for reasons
mentioned in the patch

llvm-svn: 23298