Fix target-customized spilling in the register scavenger

This is a follow-up to r178073 (which should actually make target-customized
spilling work again).

I still don't have a regression test for this (but it would be good to have
one; Thumb 1 and Mips16 use this callback as well).

Patch by Richard Sandiford.

llvm-svn: 178137