[Power9] Enable the Out-of-Order scheduling model for P9 hw

When switched to the MI scheduler for P9, the hardware is modeled as out of order.
However, inside the MI Scheduler algorithm, we still use the in-order scheduling model
as the MicroOpBufferSize isn't set. The MI scheduler take it as the hw cannot buffer
the op. So, only when all the available instructions issued, the pending instruction
could be scheduled. That is not true for our P9 hw in fact.

This patch is trying to enable the Out-of-Order scheduling model. The buffer size 44 is
picked from the P9 hw spec, and the perf test indicate that, its value won't hurt the cpu2017.

With this patch, there are 3 specs improved over 3% and 1 spec deg over 3%. The detail is as follows:

x264_r: +6.95%
cactuBSSN_r: +6.94%
lbm_r: +4.11%
xz_r: -3.85%

And the GEOMEAN for all the C/C++ spec in spec2017 is about 0.18% improved. 

Reviewer: Nemanjai
Differential Revision: https://reviews.llvm.org/D55810

llvm-svn: 350285