gallivm: handle huge number of immediates

gallivm: handle huge number of immediates

We only supported up to 256 immediates, which isn't enough. We had
code which was allocating immediates as an allocated array, but it
was always used along a statically backed array for performance
reasons. This commit adds code to skip that performance optimization
and always use just the dynamically allocated immediates if the
number of them is too great.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>