[SelectionDAG] Use int64_t to store the integer power of llvm.powi

https://llvm.org/docs/LangRef.html#llvm-powi-intrinsic
The max length of the integer power of `llvm.powi` intrinsic is 32, and
the value can be negative. If we use `int32_t` to store this value, `-Val`
will underflow when it is `INT32_MIN`

The issue was reported in D149033.