Fix interactions between 'volatile' and the Vulkan memory model

Last year we changed 'volatile' to also act as 'coherent', but when I
resolved the memory model changes against that change I missed handling
volatile in a couple places that we check for coherent. There was also
a place in post-processing that acted as if the volatile memory access
flag has a literal number associated with it, when it doesn't.

Test/spv.bufferhandle10.frag

@@ -20,4 +20,6 @@ void main() {
     coherent blockType b = t.f;
     b.x[0] = 2;
 
+    volatile blockType b2 = t.f;
+    b2.x[0] = 3;
 }