[AMDGPU] Allow use of StackPtrOffsetReg when building spills

Summary:
When spilling in the entry function we should be able to borrow
StackPtrOffsetReg as a last resort.  This restores behaviour
removed in D75138, and fixes failures when shaders use all
SGPRs, VGPRs and spill in the entry function.

Reviewers: scott.linder, arsenm, tpr

Reviewed By: scott.linder, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79776