SLPVectorizer: Clamp slp-min-reg-size to target maximum

[arsenm]

Summary

New AMDGPU hardware has 2 x 16-bit vector operations, so
a vector width of 32-bits. Currently the vector width of 32 is less
than this default of 128, the loop to pick a vector width never
executes.

Tests will be included with future backend commits.

lib/Transforms/Vectorize/SLPVectorizer.cpp

 /// Limits the size of scheduling regions in a block.
 /// It avoid long compile times for _very_ large blocks where vector
 /// instructions are spread over a wide range.
 /// This limit is way higher than needed by real-world functions.
 static cl::opt<int>
 ScheduleRegionSizeBudget("slp-schedule-budget", cl::init(100000), cl::Hidden,
     cl::desc("Limit the size of the SLP scheduling region per block"));
 
-static cl::opt<int> MinVectorRegSizeOption(
+static cl::opt<unsigned> MinVectorRegSizeOption(
     "slp-min-reg-size", cl::init(128), cl::Hidden,
     cl::desc("Attempt to vectorize for this register size in bits"));
 
 static cl::opt<unsigned> RecursionMaxDepth(
     "slp-recursion-max-depth", cl::init(12), cl::Hidden,
     cl::desc("Limit the recursion depth when building a vectorizable tree"));
 
 static cl::opt<unsigned> MinTreeSize(
BoUpSLP(Function *Func, ScalarEvolution *Se, TargetTransformInfo *Tti,
     //       data type rather than just register size. For example, x86 AVX has
     //       256-bit registers, but it does not support integer operations
     //       at that width (that requires AVX2).
     if (MaxVectorRegSizeOption.getNumOccurrences())
       MaxVecRegSize = MaxVectorRegSizeOption;
     else
       MaxVecRegSize = TTI->getRegisterBitWidth(true);
 
-    MinVecRegSize = MinVectorRegSizeOption;
+    MinVecRegSize = std::min(MinVectorRegSizeOption.getValue(), MaxVecRegSize);
   }
 
   /// \brief Vectorize the tree that starts with the elements in \p VL.
   /// Returns the vectorized root.
   Value *vectorizeTree();
   /// Vectorize the tree but with the list of externally used values \p
   /// ExternallyUsedValues. Values in this MapVector can be replaced but the
   /// generated extractvalue instructions.