[AMDGPU] Adjust wave priority based on VMEM instructions to avoid duty-cycling.

As older waves execute long sequences of VALU instructions, this may
prevent younger waves from address calculation and then issuing their
VMEM loads, which in turn leads the VALU unit to idle. This patch tries
to prevent this by temporarily raising the wave's priority.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D124246