[Asm] Add debug tracing in table-generated assembly matcher

This adds debug tracing to the table-generated assembly instruction matcher,
enabled by the -debug-only=asm-matcher option.

The changes in the target AsmParsers are to add an MCInstrInfo reference under
a consistent name, so that we can use it from table-generated code. This was
already being used this way for targets that use deprecation warnings, but 5
targets did not have it, and Hexagon had it under a different name to the other
backends.

llvm-svn: 315445