SPV: Correct selection of storage-image capabilities. Fixes #986.

Code was reflecting an old historical use of sampled as a SPIR-V
2-valued operand, instead of its current 3 values.
diff --git a/Test/baseResults/hlsl.structbuffer.fn2.comp.out b/Test/baseResults/hlsl.structbuffer.fn2.comp.out
index cb12ba5..efe5fe3 100644
--- a/Test/baseResults/hlsl.structbuffer.fn2.comp.out
+++ b/Test/baseResults/hlsl.structbuffer.fn2.comp.out
@@ -138,7 +138,7 @@
 // Id's are bound by 61
 
                               Capability Shader
-                              Capability SampledBuffer
+                              Capability ImageBuffer
                               Capability StorageImageExtendedFormats
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel Logical GLSL450