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.vec2.bracket.frag.out b/Test/baseResults/hlsl.rw.vec2.bracket.frag.out
index d920bf3..980c82b 100644
--- a/Test/baseResults/hlsl.rw.vec2.bracket.frag.out
+++ b/Test/baseResults/hlsl.rw.vec2.bracket.frag.out
@@ -1712,7 +1712,7 @@
 // Id's are bound by 605
 
                               Capability Shader
-                              Capability Sampled1D
+                              Capability Image1D
                               Capability StorageImageExtendedFormats
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel Logical GLSL450