[Assignment Tracking][24/*] Always RemoveRedundantDbgInstrs in instcombine in...

[Assignment Tracking][24/*] Always RemoveRedundantDbgInstrs in instcombine in assignment tracking builds

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

This reduces peak memory overhead by 15% when building CTMark's tramp3d-v4 with
-O2 -g with assignment tracking enabled.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133321