[PowerPC] [FastISel] i1 constants must be zero extended

When materializing constant i1 values, they must be zero extended. We represent
i1 values as [0, 1], not [0, -1], in i32 registers. As it turns out, this code
path was dead for i1 values prior to r216006 (which is why this did not manifest in
miscompiles until recently).

Fixes -O0 self-hosting on PPC64/Linux.