Rename adjustment::PointerCast and variants using it to PointerCoercion

It makes it sound like the `ExprKind` and `Rvalue` are supposed to represent all pointer related
casts, when in reality their just used to share a some enum variants. Make it clear there these
are only coercion to make it clear why only some pointer related "casts" are in the enum.

compiler/rustc_mir_transform/src/normalize_array_len.rs

@@ -41,7 +41,7 @@ fn compute_slice_length<'tcx>(
     for (local, rvalue, _) in ssa.assignments(body) {
         match rvalue {
             Rvalue::Cast(
-                CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
+                CastKind::PointerCoercion(ty::adjustment::PointerCoercion::Unsize),
                 operand,
                 cast_ty,
             ) => {