Silence warnings about -1 being converted to an unsigned value.

Silence warnings about -1 being converted to an unsigned value.

Also, pass true for isSigned even when creating constants for unsigned
comparisons, because the point is to create an all-ones constant,
rather than UINT64_MAX, even for integers wider than 64 bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102946 91177308-0d34-0410-b5e6-96231b3b80d8