[LoopSink] Use MemorySSA with legacy pass manager

LoopSink with the legacy pass manager still uses AST, because we
can't compute MemorySSA conditionally. I think now that the legacy
pass manager will be removed soon(TM) we don't need to care about
compile-time impact here anymore. Additionally, since MemorySSA is
no longer eagerly optimized, the impact is actually not that high
anymore (~0.2% geomean regression on CTMark).

This just makes legacy PM and new PM behavior line up -- as a
followup I'll drop these options entirely and make MemorySSA use
mandatory.

Differential Revision: https://reviews.llvm.org/D123216

llvm/lib/Transforms/Scalar/LoopSink.cpp

@@ -70,7 +70,7 @@ static cl::opt<bool> EnableMSSAInLoopSink(
     cl::desc("Enable MemorySSA for LoopSink in new pass manager"));
 
 static cl::opt<bool> EnableMSSAInLegacyLoopSink(
-    "enable-mssa-in-legacy-loop-sink", cl::Hidden, cl::init(false),
+    "enable-mssa-in-legacy-loop-sink", cl::Hidden, cl::init(true),
     cl::desc("Enable MemorySSA for LoopSink in legacy pass manager"));
 
 /// Return adjusted total frequency of \p BBs.

llvm/test/CodeGen/AMDGPU/opt-pipeline.ll

@@ -297,6 +297,7 @@
 ; GCN-O1-NEXT:       Basic Alias Analysis (stateless AA impl)
 ; GCN-O1-NEXT:       Function Alias Analysis Results
 ; GCN-O1-NEXT:       Scalar Evolution Analysis
+; GCN-O1-NEXT:       Memory SSA
 ; GCN-O1-NEXT:       Block Frequency Analysis
 ; GCN-O1-NEXT:       Loop Pass Manager
 ; GCN-O1-NEXT:         Loop Sink
@@ -656,6 +657,7 @@
 ; GCN-O2-NEXT:       Basic Alias Analysis (stateless AA impl)
 ; GCN-O2-NEXT:       Function Alias Analysis Results
 ; GCN-O2-NEXT:       Scalar Evolution Analysis
+; GCN-O2-NEXT:       Memory SSA
 ; GCN-O2-NEXT:       Block Frequency Analysis
 ; GCN-O2-NEXT:       Loop Pass Manager
 ; GCN-O2-NEXT:         Loop Sink
@@ -1020,6 +1022,7 @@
 ; GCN-O3-NEXT:       Basic Alias Analysis (stateless AA impl)
 ; GCN-O3-NEXT:       Function Alias Analysis Results
 ; GCN-O3-NEXT:       Scalar Evolution Analysis
+; GCN-O3-NEXT:       Memory SSA
 ; GCN-O3-NEXT:       Block Frequency Analysis
 ; GCN-O3-NEXT:       Loop Pass Manager
 ; GCN-O3-NEXT:         Loop Sink