[DSE] Allow DSE to optimize MemorySSA by default.

[fhahn]

Summary

This allows for better optimization of 'stores-of-existing-values' and
possibly helps passes further down the pipeline.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

 
 // This flags allows or disallows DSE to optimize MemorySSA during its
 // traversal. Note that DSE optimizing MemorySSA may impact other passes
 // downstream of the DSE invocation and can lead to issues not being
 // reproducible in isolation (i.e. when MemorySSA is built from scratch). In
 // those cases, the flag can be used to check if DSE's MemorySSA optimizations
 // impact follow-up passes.
 static cl::opt<bool>
-    OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(false), cl::Hidden,
-                      cl::desc("Allow DSE to optimize memory accesses"));
+    OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(true), cl::Hidden,
+                      cl::desc("Allow DSE to optimize memory accesses."));
 
 //===----------------------------------------------------------------------===//
 // Helper functions
 //===----------------------------------------------------------------------===//
 using OverlapIntervalsTy = std::map<int64_t, int64_t>;
 using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>;
 
 /// If the value of this instruction and the memory it writes to is unused, may

llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -basic-aa -dse -dse-optimize-memoryssa=false -S %s | FileCheck --check-prefixes=CHECK,UNOPT %s
 ; RUN: opt -basic-aa -dse -dse-optimize-memoryssa -S %s | FileCheck --check-prefixes=CHECK,OPT %s
-; RUN: opt -basic-aa -dse -S %s | FileCheck --check-prefixes=CHECK,UNOPT %s
+; RUN: opt -basic-aa -dse -S %s | FileCheck --check-prefixes=CHECK,OPT %s
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
 @a = external global [32 x i8], align 16
 
 declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg)
 
 ; Test case for PR16520. The store in %if.then is redundant, because the same value