Unrolled build for rust-lang#122051

Rollup merge of rust-lang#122051 - erikdesjardins:cleanup, r=nikic

cleanup: remove zero-offset GEP

This GEP would've been used to change the pointer type in the past, but after opaque pointers it's a no-op. I missed removing this in rust-lang#105545.

Split out from rust-lang#121577.

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