HLSL: allow promotion from 1-vector types to scalars, e.g, float<-float1

Previously, an error was thrown when assigning a float1 to a scalar float,
or similar for other basic types.  This allows that.

Also, this allows calling functions accepting scalars with float1 params,
so for example sin(float1) will work.  This is a minor change in
HlslParseContext::findFunction().

Test/hlsl.promote.vec1.frag

@@ -0,0 +1,16 @@
+
+float4 main() : SV_Target
+{
+    float f1a;
+    float1 f1b;
+
+    f1a = f1b;  // convert float1 to float
+    f1b = f1a;  // convert float to float1
+
+    float3 f3;
+    step(0.0, f3);
+
+    sin(f1b); // test 1-vectors in intrinsics
+
+    return float4(0,0,0,0);
+}