radeonsi/gfx9: add workarounds to avoid VGPR indexing completely

radeonsi/gfx9: add workarounds to avoid VGPR indexing completely

For inputs and outputs, indirect indexing is lowered by the GLSL compiler.
For temporaries, use alloca and disable the "promote-alloca" pass.

In the future, we could switch all codepaths to alloca permanently and
just rely on the "promote-alloca" pass.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>