AttributeSetNode: Summarize existing attributes in a bitset.

AttributeSetNode: Summarize existing attributes in a bitset.

The majority of queries just checks for the existince of an enum
attribute.  We only have 48 of those and can summaryiz them in an
uint64_t bitfield so we can avoid searching the list. This improves
"opt" compile time by 1-4% in my measurements.

Differential Revision: http://reviews.llvm.org/D16617

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259251 91177308-0d34-0410-b5e6-96231b3b80d8