Fix a hashing bug in APInt. A certain pathological testcase (too

Fix a hashing bug in APInt.  A certain pathological testcase (too
large for the testsuite) took over six minutes to compile on my Mac.
The patched LLVM-GCC compiles that testcase in three seconds (GCC
takes less than one second).  This hash function is more complex
(about 35 instructions on x86) than what Chris wanted, but I expect it
will be well-behaved with arbitrary inputs.

Thank you to everyone who responded to my previous request for advice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66962 91177308-0d34-0410-b5e6-96231b3b80d8