[KnownBits] Add fast-path for shl with unknown shift amount (NFC)

We currently don't call into KnownBits::shl() from ValueTracking
if the shift amount is unknown. If we do try to do so, we get
significant compile-time regressions, because evaluating all 64
shift amounts if quite expensive, and mostly pointless in this case.
Add a fast-path for the case where the shift amount is the full
[0, BitWidth-1] range. This primarily requires a more accurate
estimate of the max shift amount, to avoid taking the fast-path in
too many cases.

Differential Revision: https://reviews.llvm.org/D151540