[AMDGPU] Move code sinking before structurizer

Moving code sinking pass before structurizer creates more sinking
opportunities.

The extra flow edges introduced by the structurizer can have adverse
effects on sinking, because the sinking pass prefers moving instructions
to blocks with unique predecessors and the structurizer destroys that
property in some cases.

A notable example is moving high-latency image instructions across kills.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D101115