remove all-zero GEP

This always produces zero offset, regardless of what the struct layout
is.

Originally, this may have been necessary in order to change the pointer type,
but with opaque pointers, it is no longer necessary.

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -94,8 +94,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 }
 
                 if let OperandValue::Immediate(v) = cg_elem.val {
-                    let zero = bx.const_usize(0);
-                    let start = dest.project_index(bx, zero).llval;
+                    let start = dest.llval;
                     let size = bx.const_usize(dest.layout.size.bytes());
 
                     // Use llvm.memset.p0i8.* to initialize all zero arrays