[PowerPC] Use int64_t to represent stack object offset and frame size

This is the first step to enable PPC64 support huge frame size(>2G). Also fix an assertion error for frame size, i.e.,`int x; !isInt<32>(x);` should be always evaluated false, so the guard code for frame size is impossible to hit.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D107435