Auto merge of #121665 - erikdesjardins:ptradd, r=nikic

Always generate GEP i8 / ptradd for struct offsets

This implements #98615, and goes a bit further to remove `struct_gep` entirely.

Upstream LLVM is in the beginning stages of [migrating to `ptradd`](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699). LLVM 19 will [canonicalize](https://github.com/llvm/llvm-project/pull/68882) all constant-offset GEPs to i8, which has roughly the same effect as this change.

Fixes #121719.

Split out from #121577.

r? `@nikic`

compiler/rustc_codegen_llvm/src/builder.rs

@@ -603,11 +603,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                 let llptr = if i == 0 {
                     place.llval
                 } else {
-                    self.inbounds_gep(
-                        self.type_i8(),
-                        place.llval,
-                        &[self.const_usize(b_offset.bytes())],
-                    )
+                    self.inbounds_ptradd(place.llval, self.const_usize(b_offset.bytes()))
                 };
                 let llty = place.layout.scalar_pair_element_llvm_type(self, i, false);
                 let load = self.load(llty, llptr, align);
@@ -778,11 +774,6 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
-    fn struct_gep(&mut self, ty: &'ll Type, ptr: &'ll Value, idx: u64) -> &'ll Value {
-        assert_eq!(idx as c_uint as u64, idx);
-        unsafe { llvm::LLVMBuildStructGEP2(self.llbuilder, ty, ptr, idx as c_uint, UNNAMED) }
-    }
-
     /* Casts */
     fn trunc(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildTrunc(self.llbuilder, val, dest_ty, UNNAMED) }