[DebugInfo] Use a MapVector to coalesce MachineOperand locations

Summary:
The new code should be linear in the number of DBG_VALUEs, while the old
code was quadratic. NFC intended.

This is also hopefully a more direct expression of the problem, which is
to:

1. Rewrite all virtual register operands to stack slots or physical
   registers
2. Uniquely number those machine operands, assigning them location
   numbers
3. Rewrite all uses of the old location numbers in the interval map to
   use the new location numbers

In r313400, I attempted to track which locations were spilled in a
parallel bitvector indexed by location number. My code was broken
because these location numbers are not stable during rewriting.

Reviewers: aprantl, hans

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D38068

llvm-svn: 313775