AVTs are only precompiled for Literal Result Elements
via xsltCompileAttr().

For other AVTs only a check for a static value is performed via
xsltEvalStaticAttrValueTemplate().

We should precompile all AVTs with xsltCompileAttr().

Details:

For <foo bar="{foo/@bar}"/> (i.e., in Literal Result Elements), the AVT
is compiled during compilation time of the stylesheet via:
  xsltParseTemplateContent()
   --> xsltCompileAttr()
     --> xsltEvalAVT()
       --> xsltEvalXPathStringNs()
  
For <xsl:attribute name="{foo/@bar}"/>, the AVT is not precompiled,
and evaluated at transformation-time via:
  xsltAttributeInternal()
    --> xsltEvalAttrValueTemplate()
      --> xsltAttrTemplateValueProcessNode()
        --> xmlXPathCompile()
          --> xsltEvalXPathStringNs()