i965: Fix sampler state pointer adjustment for nonconst samplers

This started hitting an assertion recently. Only affects Haswell
(Ivybridge doesn't support this meddling with the sampler state pointer,
and ARB_gpu_shader5 is not enabled yet on Broadwell)

14 Piglits crash->pass.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
util: Implement unreachable for MSVC using __assume

Based on the description of __assume at:

http://msdn.microsoft.com/en-us/library/1b3fsfxw.aspx

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
mesa: Silence unused parameter warning in check_context_limits in non-debug builds

../../src/mesa/main/context.c: In function 'check_context_limits':
../../src/mesa/main/context.c:733:41: warning: unused parameter 'ctx' [-Wunused-parameter]

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>