Re-land "[DebugInfo] Insert DW_OP_deref when spilling indirect DBG_VALUEs"

After r313775, it's easier to maintain a parallel BitVector of spilled
locations indexed by location number.

I wasn't able to build a good reduced test case for this iteration of
the bug, but I added a more direct assertion that spilled values must
use frame index locations. If this bug reappears, it won't only fire on
the NEON vector code that we detected it on, but on medium-sized
integer-only programs as well.

llvm-svn: 313786