[DAG] Do not call computeKnownBits from isKnownToBeAPowerOfTwo

The only way known bits could help identify a known power of two is if
it knows exactly which power of two it is, i.e. if it is a known
constant. But in that case the value should have been simplified to a
constant already. So save some compile time by not calling
computeKnownBits.

Differential Revision: https://reviews.llvm.org/D149325