ValueTracking: Use fixed array for assumption exclude set in Query.

The Query structure is constructed often and is relevant for compiletime
performance. We can replace the SmallPtrSet for assumption exclusions in
this structure with a fixed size array because we know the maximum
number of elements.  This improves typical clang -O3 -emit-llvm compiletime
by 1.2% in my measurements.

Differential Revision: http://reviews.llvm.org/D16204

llvm-svn: 259025