Check existence of each required component during construction of LLVMCDisassembler.

Summary:
Actually, fix two issues:

  # remove repeat creation of reg_info, use m_reg_info_ap  for createMCAsmInfo instead;

  # remove possibility to dereference nullptr during createMCAsmInfo invocation, that could lead to undefined behavior.

Placed checking of a component right after its creation to simplify the code and avoid same issues later.

Reviewers: zturner, clayborg, jingham, jasonmolenda, labath

Reviewed By: clayborg, labath

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D41584

llvm-svn: 322270