[PPC] Use the correct immediate operands on 64-bit instructions

Several of the 64-bit fixed-point instructions with immediate operands were
using the 32-bit (i32) operand nodes instead of the corresponding 64-bit (i64)
operand definitions (u16imm instead of u16imm64, for example).

This error has had no effect so far, but would have caused type-checking
violations with an upcoming change.

llvm-svn: 198356