[LoopSink] Use MemorySSA with legacy pass manager

[nikic]

Summary

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% regression: http://llvm-compile-time-tracker.com/compare.php?from=ed4e6e03981a37d54c4ac635b2bdb2c2fe8fec92&to=c3bfd875b637503adbeaccc59d19d8eb934bcfa7&stat=instructions)

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.

[nikic]

Update pipeline test

[fhahn]

LGTM, dropping the non-MSSA code path as follow up sounds good!

[asbirlea]

+1 to the additional cleanup.

llvm/lib/Transforms/Scalar/LoopSink.cpp

static cl::opt<unsigned> MaxNumberOfUseBBsForSinking(
     "max-uses-for-sinking", cl::Hidden, cl::init(30),
     cl::desc("Do not sink instructions that have too many uses."));
 
 static cl::opt<bool> EnableMSSAInLoopSink(
     "enable-mssa-in-loop-sink", cl::Hidden, cl::init(true),
     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.
 ///
 /// * If there is only one BB, sinking instruction will not introduce code
 ///   size increase. Thus there is no need to adjust the frequency.
 /// * If there are more than one BB, sinking would lead to code size increase.
 ///   In this case, we add some "tax" to the total frequency to make it harder