Change APInt comparison with uint64_t.

Summary:
This patch changes the way APInt is compared with a value of type uint64_t.
Before the uint64_t value was truncated to the size of APInt before comparison.
Now the comparison takes into account full 64-bit precision.

Test Plan: Unit tests added. No regressions. Self-hosted check-all done as well.

Reviewers: chandlerc, dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10655

llvm-svn: 241204