APInt: Simplify EqualSlowCase

Previously the code used getActiveBits() to determine the highest set
bit of each APInt first. However doing so requires the same amount of
memory accesses as simply comparing both numbers right away.

Removing all the active bit checks leads to simpler code and is faster
in my benchmark.

Differential Revision: http://reviews.llvm.org/D16620

llvm-svn: 260447