[intelpt] Refactoring instruction decoding for flexibility

[intelpt] Refactoring instruction decoding for flexibility

Now the decoded thread has Append methods that provide more flexibility
in terms of the underlying data structure that represents the
instructions. In this case, we are able to represent the sporadic errors
as map and thus reduce the size of each instruction.

Differential Revision: https://reviews.llvm.org/D122293