[RemoveRedundantDebugValues] Add a Pass that removes redundant DBG_VALUEs

This new MIR pass removes redundant DBG_VALUEs.

After the register allocator is done, more precisely, after
the Virtual Register Rewriter, we end up having duplicated
DBG_VALUEs, since some virtual registers are being rewritten
into the same physical register as some of existing DBG_VALUEs.
Each DBG_VALUE should indicate (at least before the LiveDebugValues)
variables assignment, but it is being clobbered for function
parameters during the SelectionDAG since it generates new DBG_VALUEs
after COPY instructions, even though the parameter has no assignment.
For example, if we had a DBG_VALUE $regX as an entry debug value
representing the parameter, and a COPY and after the COPY,
DBG_VALUE $virt_reg, and after the virtregrewrite the $virt_reg gets
rewritten into $regX, we'd end up having redundant DBG_VALUE.

This breaks the definition of the DBG_VALUE since some analysis passes
might be built on top of that premise..., and this patch tries to fix
the MIR with the respect to that.

This first patch performs bacward scan, by trying to detect a sequence of
consecutive DBG_VALUEs, and to remove all DBG_VALUEs describing one
variable but the last one:

For example:

(1) DBG_VALUE $edi, !"var1", ...
(2) DBG_VALUE $esi, !"var2", ...
(3) DBG_VALUE $edi, !"var1", ...
 ...

in this case, we can remove (1).

By combining the forward scan that will be introduced in the next patch
(from this stack), by inspecting the statistics, the RemoveRedundantDebugValues
removes 15032 instructions by using gdb-7.11 as a testbed.

Differential Revision: https://reviews.llvm.org/D105279