X86AsmBackend: Remove some computeFragmentSize calls
The function was brittle as it performed only one iteration, which might
not converge. The reverted be5a845e4c29aadb513ae6e5e2879dccf37efdbb
mentioned that !NDEBUG and NDEBUG builds evaluated fragment offsets at
different times.
* X86AsmBackend::finishLayout performed only one iteration, which might
not converge. In addition, the removed `#ifndef NDEBUG` code (disabled
by default) in X86AsmBackend::finishLayout was problematic, as !NDEBUG
and NDEBUG builds evaluated fragment offsets at different times before
this patch.
