Assigning a Builtin to Itself Leads to Panic

[cyberthirst]

Version Information

• vyper Version (output of vyper --version): commit 4b4e188ba83d28b5dd6ff66479e7448e5b925030

Issue description

Assigning some builtin to itself is not prevented and leads to a
compiler panic.

POC

For example, compiling the following code leads the compiler to raise
the following exception: CodegenPanic: unhandled exception , parse_Name.

@external
def foo():
    convert = convert

credits: @trocher