[trace][intel pt] Handle better tsc in the decoder

A problem that I introduced in the decoder is that I was considering TSC decoding
errors as actual instruction errors, which mean that the trace has a gap. This is
wrong because a TSC decoding error doesn't mean that there's a gap in the trace.
Instead, now I'm just counting how many of these errors happened and I'm using
the `dump info` command to check for this number.

Besides that, I refactored the decoder a little bit to make it simpler, more
readable, and to handle TSCs in a cleaner way.

Differential Revision: https://reviews.llvm.org/D122867