[MCTargetAsmParser] Move the member varialbes that reference

[MCTargetAsmParser] Move the member varialbes that reference
MCSubtargetInfo in the subclasses into MCTargetAsmParser and define a
member function getSTI.

This is done in preparation for making changes to shrink the size of
MCRelaxableFragment. (see http://reviews.llvm.org/D14346).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253124 91177308-0d34-0410-b5e6-96231b3b80d8