[AMDGPU] Split the lit test spill-vgpr-to-agpr.ll to different tests

[1]. Move the test which reject the usage of agpr before gfx908 into a separate file - reject-agpr-usage-before-gfx908.ll.
[2]. Move those tests which are applicable to both gfx900 and gfx908 into a separate file - spill-vgpr.ll.
[3]. Keep those tests which are specific to only gfx908 in the file spill-vgpr-to-agpr.ll.

Above split is required to properly update the tests in D123525.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D123973