On the vec4 backend, textureSamplesIdentical() will always return
false.  There are currently no test cases for the vec4 backend, so we
don't have much confidence in any implementation.  We also don't think
anyone is likely to miss it.

v2: Handle immediate value for MCS smarter.  Rebase on changes to
nir_texop_sampels_identical (missing second parameter).  Suggested by
Jason.

v3: Add Neil's code to handle 16x MSAA in the FS.  Also rebase on top of
f9a9ba5e

.  Stub out the vec4 implementation.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com> [v2]
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz> [v2]