[ARM] Fix "Constant pool entry out of range!" in Thumb1 mode

This patch fixes PR36658, "Constant pool entry out of range!" in Thumb1 mode.

In ARMConstantIslands::optimizeThumb2JumpTables() in Thumb1 mode,
adjustBBOffsetsAfter() is not calculating postOffset correctly by
properly accounting for the padding that is required for the constant pool
that immediately follows the jump table branch  instruction.

Reviewers: t.p.northover, eli.friedman

Reviewed By: t.p.northover

Subscribers: chrib, tstellar, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D44709

llvm-svn: 328341