[AMDGPU] Untangle SDWA pass from SIShrinkInstructions

[AMDGPU] Untangle SDWA pass from SIShrinkInstructions

Remove dependency of SDWA pass on SIShrinkInstructions.
The goal is to move SDWA even higher in the stack to avoid second run
of MachineLICM, MachineCSE and SIFoldOperands.

Also added handling to preserve original src modifiers.

Differential Revision: https://reviews.llvm.org/D33860

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304665 91177308-0d34-0410-b5e6-96231b3b80d8