[llvm-exegesis] Factor out DisassemblerHelper from the Analysis class

[llvm-exegesis] Factor out DisassemblerHelper from the Analysis class

As part of preparing the reports, the Analysis class needs to print
machine instructions in a disassembled form. For this purpose, the class
has four fields (namely Context_, AsmInfo_, InstPrinter_ and Disasm_).
All the constructor of the Analysis class does is conditionally
initializing these four fields.

This commit factors out the logic for decoding machine code and printing
it in an assembler form into a separate DisassemblerHelper class.

~~

Huawei RRI, OS Lab

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D147156