Add a new function tcExtract for extracting a bignum from an

arbitrary range of bits embedded in the middle of another bignum.
This kind of operation is desirable in many cases of software
floating point, e.g. converting bignum integers to floating point
numbers of fixed precision (you want to extract the precision most
significant bits).

Elsewhere, add an assertion, and exit the shift functions early if
the shift count is zero.

llvm-svn: 42745