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.

llvm-svn: 66962