[LV] Wrap LV illegality reporting in a function. NFC.

A function for loop vectorization illegality reporting has been
introduced:

void LoopVectorizationLegality::reportVectorizationFailure(
    const StringRef DebugMsg, const StringRef OREMsg,
    const StringRef ORETag, Instruction * const I) const;

The function prints a debug message when the debug for the compilation
unit is enabled as well as invokes the optimization report emitter to
generate a message with a specified tag. The function doesn't cover any
complicated logic when a custom lambda should be passed to the emitter,
only generating a message with a tag is supported.

The function always prints the instruction `I` after the debug message
whenever the instruction is specified, otherwise the debug message
ends with a dot: 'LV: Not vectorizing: Disabled/already vectorized.'

Patch by Pavel Samolysov <samolisov@gmail.com>

llvm-svn: 362736