nir: Use a bitfield for image access qualifiers

This commit expands the current memory access enum to contain the extra
two bits provided for images.  We choose to follow the SPIR-V convention
of NonReadable and NonWriteable because readonly implies that you *can*
read so readonly + writeonly doesn't make as much sense as NonReadable +
NonWriteable.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/shader_enums.h

@@ -690,11 +690,13 @@ enum gl_frag_depth_layout
 /**
  * \brief Buffer access qualifiers
  */
-enum gl_buffer_access_qualifier
+enum gl_access_qualifier
 {
-   ACCESS_COHERENT = 1,
-   ACCESS_RESTRICT = 2,
-   ACCESS_VOLATILE = 4,
+   ACCESS_COHERENT      = (1 << 0),
+   ACCESS_RESTRICT      = (1 << 1),
+   ACCESS_VOLATILE      = (1 << 2),
+   ACCESS_NON_READABLE  = (1 << 3),
+   ACCESS_NON_WRITEABLE = (1 << 4),
 };
 
 /**