[ARM] Fix selection of VLDR.16 instruction with imm offset

The isScaledConstantInRange function takes upper and lower bounds which are
checked after dividing by the scale, so the bounds checks for half, single and
double precision should all be the same. Previously, we had wrong bounds checks
for half precision, so selected an immediate the instructions can't actually
represent.

Differential revision: https://reviews.llvm.org/D58822

llvm-svn: 355305