Fast-track obviously over-large and over-small exponents during decimal->

Fast-track obviously over-large and over-small exponents during decimal->
integer conversion.  In some such cases this makes us one or two orders
of magnitude faster than NetBSD's libc.  Glibc seems to have a similar
fast path.

Also, tighten up some upper bounds to save a bit of memory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42984 91177308-0d34-0410-b5e6-96231b3b80d8