Reduce the size of MCRelaxableFragment.

Reduce the size of MCRelaxableFragment.

MCRelaxableFragment previously kept a copy of MCSubtargetInfo and
MCInst to enable re-encoding the MCInst later during relaxation. A copy
of MCSubtargetInfo (instead of a reference or pointer) was needed
because the feature bits could be modified by the parser.

This commit replaces the MCSubtargetInfo copy in MCRelaxableFragment
with a constant reference to MCSubtargetInfo. The copies of
MCSubtargetInfo are kept in MCContext, and the target parsers are now
responsible for asking MCContext to provide a copy whenever the feature
bits of MCSubtargetInfo have to be toggled.

With this patch, I saw a 4% reduction in peak memory usage when I
compiled verify-uselistorder.lto.bc using llc.

rdar://problem/21736951

Differential Revision: http://reviews.llvm.org/D14346

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253127 91177308-0d34-0410-b5e6-96231b3b80d8