[LoopVectorize] Don't unconditionally print vectorization diagnostics

when compiling with LTO.

r244523 a new class DiagnosticInfoOptimizationRemarkAnalysisAliasing for
optimization analysis remarks related to pointer aliasing without
guarding it in isDiagnosticEnabled in LLVMContext.cpp. This caused the
diagnostic message to be printed unconditionally when compiling with
LTO.

This commit cleans up isDiagnosticEnabled and makes sure all the
vectorization optimization remarks are guarded.

rdar://problem/25382153

llvm-svn: 265084