AMDGPU/SI: Do not move scratch resource register on Tonga & Iceland

Due to the SGPR init bug, every program claims to use the same number
of SGPRs anyway, so there's no point in trying to shift those registers
down from their initial spot of reservation.

Add a test that uses VGPR spilling and blocks most SGPRs from being used for
the scratch resource register. Previously, this would run into an assertion.

Differential Revision: http://reviews.llvm.org/D15724

llvm-svn: 256870