[AMDGPU] Switch to backwards scavenging in eliminateFrameIndex

Frame index elimination runs backwards so we must use backwards
scavenging. Otherwise, when a scavenged register is spilled, the
scavenger will remember that the register is in use until the restore
point, but it will never reach that restore point. The result is that in
some cases it will keep scavenging different registers instead of
reusing the same one.

Differential Revision: https://reviews.llvm.org/D152394