[2/2][RemoveRedundantDebugValues] Add a Pass that removes redundant DBG_VALUEs

[2/2][RemoveRedundantDebugValues] Add a Pass that removes redundant DBG_VALUEs

This patch adds the forward scan for finding redundant DBG_VALUEs.

This analysis aims to remove redundant DBG_VALUEs by going forward
in the basic block by considering the first DBG_VALUE as a valid
until its first (location) operand is not clobbered/modified.
For example:

(1) DBG_VALUE $edi, !"var1", ...
(2) <block of code that does affect $edi>
(3) DBG_VALUE $edi, !"var1", ...
 ...
in this case, we can remove (3).

Differential Revision: https://reviews.llvm.org/D105280