[PowerPC] Fix incorrectly encoded wait instruction
Encoding for the wait instruction was wrong. Fix according to ISA 3.0.
Differential Revision: https://reviews.llvm.org/D48550
Has this been tested out on Power8/Power7? The encoding according to ISA 2.07 is:
31 /// WC /// /// 62 /
I am not sure why this has changed since ISA 2.07. But I have verified the new encoding on pwr8/pwr9 and that the old encoding produces an illegal instruction.