aco: Implement subgroup shuffle in GFX10 wave64 mode.

aco: Implement subgroup shuffle in GFX10 wave64 mode.

Previously subgroup shuffle was implemented using the bpermute
instruction, which only works accross half-waves, so by itself it's
not suitable for implementing subgroup shuffle when the shader is
running in wave64 mode.

This commit adds a trick using shared VGPRs that allows to implement
subgroup shuffle still relatively effectively in this mode.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>