HLSL: Implicit bool conversions for conditional expressions and related.

Covers if(cond), while(cond), do-while(cond), for(;cond;), and (cond ? :).
Fixes #778.

Test/hlsl.implicitBool.frag

@@ -0,0 +1,32 @@
+float condf;
+int condi;
+float1 condf1;
+int1 condi1;
+
+float4 main() : SV_Target0
+{
+    float4 a = float4(2.0, 2.0, 2.0, 2.0);
+    if (condi)
+        return a + 1.0;
+    if (condf)
+        return a + 2.0;
+    if (condf1)
+        return a + 3.0;
+    if (condi1)
+        return a + 4.0;
+    if (condi && condf || condf1)
+        return a + 5.0;
+
+    float f = condf;
+    while (f) { --f; }
+
+    int i = condi;
+    do { --i; } while (i);
+
+    for (; i; ) { --i; }
+
+    float g = condf ? 7.0 : 8.0;
+    a += g;
+
+    return a - 1.0;
+}