[AMDGPU] Fix VGPR spills where offset doesn't fit in 12 bits

Scale the offset of VGPR spills by the wave size when it cannot fit in the
12-bit offset immediate field and so is added to the soffset SGPR. This
accounts for hardware swizzling of scratch memory.

Differential Revision: https://reviews.llvm.org/D49448

llvm-svn: 338060