Avoid abort on "memory read -s N" for N=3,5,6,7

We cannot use "GetMaxU64Bitfield" for non-power-of-two sizes, so just use
the same code that handles N > 8 for these.

Review: http://llvm-reviews.chandlerc.com/D1699


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@190873 91177308-0d34-0410-b5e6-96231b3b80d8