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.rw.scalar.bracket.frag.out b/Test/baseResults/hlsl.rw.scalar.bracket.frag.out
index da7a5a1..7ff34fa 100644
--- a/Test/baseResults/hlsl.rw.scalar.bracket.frag.out
+++ b/Test/baseResults/hlsl.rw.scalar.bracket.frag.out
@@ -1694,7 +1694,7 @@
 // Id's are bound by 571
 
                               Capability Shader
-                              Capability Sampled1D
+                              Capability Image1D
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel Logical GLSL450
                               EntryPoint Fragment 4  "main" 547