[AMDGPU] Reserving VGPR for future SGPR Spill

Summary: One VGPR register is allocated to handle a future spill of SGPR if "--amdgpu-reserve-vgpr-for-sgpr-spill" option is used

Reviewers: arsenm, rampitec, msearles, cdevadas

Reviewed By: arsenm

Subscribers: madhur13490, qcolombet, kerbowa, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #amdgpu, #llvm

Differential Revision: https://reviews.llvm.org/D70379