[LV] Wrap LV illegality reporting in a function. NFC.
A function for loop vectorization illegality reporting has been
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 <email@example.com>