Fix branch relaxation in 16-bit mode.

Thread through MCSubtargetInfo to relaxInstruction function allowing relaxation
to generate jumps with 16-bit sized immediates in 16-bit mode.

This fixes PR22097.

Reviewers: dwmw2, tstellarAMD, craig.topper, jyknight

Subscribers: jfb, arsenm, jyknight, llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D20830

llvm-svn: 275068