[X86] Remove an unnecessary 'if' that prevented treating INT64_MAX and...

[X86] Remove an unnecessary 'if' that prevented treating INT64_MAX and -INT64_MAX as power of 2 minus 1 in the multiply expansion code.

Not sure why they were being explicitly excluded, but I believe all the math inside the if works. I changed the absolute value to be uint64_t instead of int64_t so INT64_MIN+1 wouldn't be signed wrap.

llvm-svn: 338101