Add IntLog2 and use it

Replace uses of floating point log2 when we want an integer result from
an integer operand.  This avoids concerns about accuracy of floating
point library functions.

glslang/Include/Common.h

@@ -286,6 +286,18 @@ template <class T> bool IsMultipleOfPow2(T number, int powerOf2)
     return ! (number & (powerOf2 - 1));
 }
 
+// Returns log2 of an integer power of 2.
+// T should be integral.
+template <class T> int IntLog2(T n)
+{
+    assert(IsPow2(n));
+    int result = 0;
+    while ((T(1) << result) != n) {
+      result++;
+    }
+    return result;
+}
+
 } // end namespace glslang
 
 #endif // _COMMON_INCLUDED_