The VGPRs used for SGPR spills need to be reserved, even if we aren't
speculatively reserving one.
This was broken by 117e5609e98b43f925c678b72f816ad3a1c3eee7.
In D89510#2333593, @rampitec wrote:
We apparently don't have any tests turning off the pre-reserved VGPR. With the getReservedRegs change, one test fails due to reserveVGPRforSGPRSpills returning NoRegister