Use size_of_val instead of manual calculation

Very minor thing that I happened to notice in passing, but it's both shorter and means it gets `mul nuw`, so why not.

compiler/rustc_codegen_ssa/src/glue.rs

@@ -46,7 +46,7 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                 // NOTE: ideally, we want the effects of both `unchecked_smul` and `unchecked_umul`
                 // (resulting in `mul nsw nuw` in LLVM IR), since we know that the multiplication
                 // cannot signed wrap, and that both operands are non-negative. But at the time of writing,
-                // `BuilderMethods` can't do this, and it doesn't seem to enable any further optimizations.
+                // the `LLVM-C` binding can't do this, and it doesn't seem to enable any further optimizations.
                 bx.unchecked_smul(info.unwrap(), bx.const_usize(unit.size.bytes())),
                 bx.const_usize(unit.align.abi.bytes()),
             )

library/core/src/hash/mod.rs

@@ -834,7 +834,7 @@ mod impls {
 
                 #[inline]
                 fn hash_slice<H: ~const Hasher>(data: &[$ty], state: &mut H) {
-                    let newlen = data.len() * mem::size_of::<$ty>();
+                    let newlen = mem::size_of_val(data);
                     let ptr = data.as_ptr() as *const u8;
                     // SAFETY: `ptr` is valid and aligned, as this macro is only used
                     // for numeric primitives which have no padding. The new slice only