[AMDGPU] Move SiShrinkInstruction and SDWAPeephole to SSAOptimization passes

Summary:
Difference beetween PreRegAlloc() and MachineSSAOptimization() are that the former is run despite of -O0 optimization level. In my undestanding SiShrinkInstructions and SDWAPeephole shouldn't run when optimizations are disabled.
With this change order of passes will not change.

Reviewers: arsenm, vpykhtin, rampitec

Subscribers: qcolombet, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D31705

llvm-svn: 299757