[CGP] Fix crash on i96 bit multiply

Issue found by llvm-isel-fuzzer on OSS fuzz, https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3725

If anyone actually cares about > 64 bit arithmetic, there's a lot more to do in this area.  There's a bunch of obviously wrong code in the same function.  I don't have the time to fix all of them and am just using this to understand what the workflow for fixing fuzzer cases might look like.

llvm-svn: 316967