[AMDGPU] Correct the setting of `FlatScratchInit`.

Summary: - That flag setting should skip spilling stack slot.

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64143

llvm-svn: 365137