[pred-memopt] Replace remaining occurances of prefix DI with PMO prefix.

I am going to DI and predictable mem opts have been split for a long time and
their subroutines aren't going to be joined in the future... so replace the DI
prefixes in pred-mem-opts impl with PMO and rename DIMemoryUseCollector =>
PMOUseCollector.

Been sitting on this for a long time... just happy to get it in.

(cherry picked from commit db30959)

Author: gottesmm

lib/SILOptimizer/Mandatory/CMakeLists.txt

@@ -4,7 +4,6 @@ set(MANDATORY_SOURCES
   Mandatory/AddressLowering.cpp
   Mandatory/ConstantPropagation.cpp
   Mandatory/DefiniteInitialization.cpp
-  Mandatory/DIMemoryUseCollector.cpp
   Mandatory/DIMemoryUseCollectorOwnership.cpp
   Mandatory/DataflowDiagnostics.cpp
   Mandatory/DiagnoseInfiniteRecursion.cpp
@@ -14,6 +13,7 @@ set(MANDATORY_SOURCES
   Mandatory/IRGenPrepare.cpp
   Mandatory/MandatoryInlining.cpp
   Mandatory/PredictableMemOpt.cpp
+  Mandatory/PMOMemoryUseCollector.cpp
   Mandatory/SemanticARCOpts.cpp
   Mandatory/ClosureLifetimeFixup.cpp
   Mandatory/RawSILInstLowering.cpp