[Support] Add KnownBits::countMaxSignedBits(). Make...

[Support] Add KnownBits::countMaxSignedBits(). Make KnownBits::countMinSignBits() always return at least 1.

Even if we don't have any known bits, we can assume that there is
at least 1 sign bit. This is consistent with ComputeNumSignBits
which always returns at least 1.

Add KnownBits::countMaxSignedBits() which computes the number of
bits needed to represent all signed values with those known bits.
This is the signed equivalent of countMaxActiveBits().

Split from D116469.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D116500