[AMDGPU] Make temp vgpr selection stable in indirectCopyToAGPR

This uses rotating reminder of division by 3 to select another
temp vgpr each next time in a sequence of several agpr copies.
Therefore, temp vgpr selection depends on the generated agpr
number. This number could change with any unrelated change to
the register definitions.

Stabilize the selection by using a real agpr number.

Differential Revision: https://reviews.llvm.org/D127524