SPV: Implement short circuiting of && and || when emitting SPIR-V.

Test/spv.shortCircuit.frag

@@ -0,0 +1,50 @@
+#version 400
+
+uniform ivec4 uiv4;
+uniform vec4 uv4;
+uniform bool ub;
+uniform bool uba;
+uniform bvec4 ub41, ub42;
+uniform float uf;
+uniform int ui;
+
+out float of1;
+out vec4  of4;
+
+bool foo() { ++of1; return of1 > 10.0; }
+
+void main()
+{
+    of1 = 0.0;
+    of4 = vec4(0.0);
+
+    if (ub || ui > 2)  // not worth short circuiting
+        ++of1;
+
+    if (ub && !uba)  // not worth short circuiting
+        ++of1;
+
+    if (ub || foo())   // must short circuit
+        ++of1;
+
+    if (ub && foo())   // must short circuit
+        ++of1;
+
+    if (foo() || ub)   // not worth short circuiting
+        ++of1;
+
+    if (foo() && ub)   // not worth short circuiting
+        ++of1;
+
+    if (ub || ++of1 > 1.0)   // must short circuit
+        ++of4;
+
+    if (++of1 > 1.0 || ub)   // not worth short circuiting
+        ++of4;
+
+    if (ub || sin(uf) * 4.0 > of1)  // worth short circuiting
+        ++of1;
+
+    if (ub && sin(uf) * 4.0 > of1)  // worth short circuiting
+        ++of1;
+}