[CodeGen] Add standard print/debug utilities to MVT

Doing so makes it easier to do printf style debugging in idiomatic manner. I followed the code structure of Value with only the definition of dump being #ifdef out in non-debug builds. Not sure if this is the "right" option; we don't seem to have any single consistent scheme on how dump is handled.

Note: This is a follow up to D143454 which did the same for EVT.

Differential Revision: https://reviews.llvm.org/D143511