[PowerPC] fix latency for simple integer instructions in POWER9 scheduler

In the POWER9 instruction scheduler, SchedWriteRes for the simple integer instructions are misconfigured to use that of (costly) DFU instructions.
This results in surprisingly long instruction latency estimation and causes misbehavior in some optimizers such as if-conversion.

Differential Revision: https://reviews.llvm.org/D34869

llvm-svn: 307624