[ARM] Fix FP16 stack loads/stores for Thumb2 with frame pointer

The new addressing mode added for the v8.2A FP16 instructions uses bit 8 of the
immediate to encode the sign of the offset, like the other FP loads/stores, so
need to be treated the same way.

Differential revision: https://reviews.llvm.org/D58816

llvm-svn: 355201