[LoopVectorize] Rename the Report class to VectorizationReport

This class will become public in the new LoopAccessAnalysis header so the name
needs to be more global.

NFC.  This is part of the patchset that splits out the memory dependence logic
from LoopVectorizationLegality into a new class LoopAccessAnalysis.
LoopAccessAnalysis will be used by the new Loop Distribution pass.

llvm-svn: 227749