[ConstantRange] Add fast signed multiply

The multiply() implementation is very slow -- it performs six
multiplications in double the bitwidth, which means that it will
typically work on allocated APInts and bypass fast-path
implementations. Add an additional implementation that doesn't
try to produce anything better than a full range if overflow is
possible. At least for the BasicAA use-case, we really don't care
about more precise modeling of overflow behavior. The current
use of multiply() is fine while the implementation is limited to
a single index, but extending it to the multiple-index case makes
the compile-time impact untenable.