nir: Use alloca instead of variable length arrays.

nir: Use alloca instead of variable length arrays.

This is to enable the code to build with -Werror=vla in the short term,
and enable the code to build with MSVC2013 soon after.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>