Change cg_ssa's get_param to borrow the builder mutably
This is a small change to make `get_param` more flexible for codegens that may need to modify things when retrieving function parameters.
This will currently only be used by [rustc_codegen_nvvm](https://github.com/Rust-GPU/Rust-CUDA) (my own project), but may be useful to more codegens in the future.
This is needed because cg_nvvm needs to remap certain types to libnvvm-friendly types, such as `i128` -> `<2 x i64>`. Because cg_ssa does not give mutable access to the builder, i resorted to using a mutex:
```rs
fn get_param(&self, index: usize) -> Self::Value {
let val = llvm::get_param(self.llfn(), index as c_uint);
trace!("Get param `{:?}`", val);
unsafe {
let llfnty = LLVMRustGetFunctionType(self.llfn());
let map = self.remapped_integer_args.borrow();
if let Some((_, key)) = map.get(llfnty) {
if let Some((_, new_ty)) = key.iter().find(|t| t.0 == index) {
trace!("Casting irregular param {:?} to {:?}", val, new_ty);
return transmute_llval(
*self.llbuilder.lock().unwrap(),
&self.cx,
val,
*new_ty,
);
}
}
val
}
}
```
However, i predict this is pretty bad for performance, considering how much builders are called during codegen, so i would greatly appreciate having a more flexible API for this.
324b4bcb3c