IR: make getParamByValType Just Work. NFC.

Most parts of LLVM don't care whether the byval type is derived from an
explicit Attribute or from the parameter's pointee type, so it makes
sense for the main access function to just return the right value.

The very few users who do care (only BitcodeReader so far) can find out
how it's specified by accessing the Attribute directly.

llvm-svn: 362642