[mlir] Document priority of alignment options.

Alignment can be specified either explicitly via
attributes in vector operations or via the option
to use-vector-alignment=<N> in the --convert-vector-to-llvm
pass. The explicit attribute takes precedent over the option
used as input to the pass.

Author: amd-eochoalo

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -247,6 +247,7 @@ class VectorLoadStoreConversion : public ConvertOpToLLVMPattern<LoadOrStoreOp> {
     MemRefType memRefTy = loadOrStoreOp.getMemRefType();
 
     // Resolve alignment.
+    // Explicit alignment takes priority over use-vector-alignment.
     unsigned align = loadOrStoreOp.getAlignment().value_or(0);
     if (!align &&
         failed(getVectorToLLVMAlignment(*this->getTypeConverter(), vectorTy,
@@ -299,6 +300,7 @@ class VectorGatherOpConversion
     }
 
     // Resolve alignment.
+    // Explicit alignment takes priority over use-vector-alignment.
     unsigned align = gather.getAlignment().value_or(0);
     if (!align &&
         failed(getVectorToLLVMAlignment(*this->getTypeConverter(), vType,
@@ -355,6 +357,7 @@ class VectorScatterOpConversion
     }
 
     // Resolve alignment.
+    // Explicit alignment takes priority over use-vector-alignment.
     unsigned align = scatter.getAlignment().value_or(0);
     if (!align &&
         failed(getVectorToLLVMAlignment(*this->getTypeConverter(), vType,