[MC] Move bundling and MCSubtargetInfo to MCEncodedFragment [NFC]

Instruction bundling is only supported on descendants of the
MCEncodedFragment type. By moving the bundling functionality and
MCSubtargetInfo to this class it makes it easier to set and extract the
MCSubtargetInfo when it is necessary.

This is a refactoring change that will make it easier to pass the
MCSubtargetInfo through to writeNops when nop padding is required.

Differential Revision: https://reviews.llvm.org/D45959

llvm-svn: 334814