[ARM] Fix offset calculation in ARMBaseRegisterInfo::needsFrameBaseReg

The input offset to needsFrameBaseReg is a negative value below the top of the
stack frame, but when converting to a positive offset from the bottom of the
stack frame this value was negated, causing the final offset to be too large
by twice the input offset's magnitude. Fix that by not negating the offset.

Patch by John Brawn

Differential Revision: http://reviews.llvm.org/D8316

llvm-svn: 232513