AMDGPU: Correct const_index_stride for wave 32 for PAL ABI

Since there is a single scratch resource descriptor for all shaders, if there is
a wave32 and a wave64 shader (for instance for VsFs pairs)
then the const_index_stride will be incorrect for wave32 shaders.

Differential Revision: https://reviews.llvm.org/D101830

Change-Id: Id8de5566b0d1a07a814e2e7db016df9d20bf6d2c