Tighten up assignment operator representations.

In the AST, currently we use `BinOpKind` within `ExprKind::AssignOp` and
`AssocOp::AssignOp`, even though this allows some nonsensical
combinations. E.g. there is no `&&=` operator. Likewise for HIR and
THIR.

This commit introduces `AssignOpKind` which only includes the ten
assignable operators, and uses it in `ExprKind::AssignOp` and
`AssocOp::AssignOp`. (And does similar things for `hir::ExprKind` and
`thir::ExprKind`.) This avoids the possibility of nonsensical
combinations, as seen by the removal of the `bug!` case in
`lang_item_for_binop`.

The commit is mostly plumbing, including:
- Adds an `impl From<AssignOpKind> for BinOpKind` (AST) and `impl
  From<AssignOp> for BinOp` (MIR/THIR).
- `BinOpCategory` can now be created from both `BinOpKind` and
  `AssignOpKind`.
- Replaces the `IsAssign` type with `Op`, which has more information and
  a few methods.
- `suggest_swapping_lhs_and_rhs`: moves the condition to the call site,
  it's easier that way.
- `check_expr_inner`: had to factor out some code into a separate
  method.

I'm on the fence about whether avoiding the nonsensical combinations is
worth the extra code.

compiler/rustc_ast/src/util/parser.rs

@@ -1,6 +1,6 @@
 use rustc_span::kw;
 
-use crate::ast::{self, BinOpKind, RangeLimits};
+use crate::ast::{self, AssignOpKind, BinOpKind, RangeLimits};
 use crate::token::{self, Token};
 
 /// Associative operator.
@@ -9,7 +9,7 @@ pub enum AssocOp {
     /// A binary op.
     Binary(BinOpKind),
     /// `?=` where ? is one of the assignable BinOps
-    AssignOp(BinOpKind),
+    AssignOp(AssignOpKind),
     /// `=`
     Assign,
     /// `as`
@@ -44,16 +44,16 @@ impl AssocOp {
             token::Or => Some(Binary(BinOpKind::BitOr)),
             token::Shl => Some(Binary(BinOpKind::Shl)),
             token::Shr => Some(Binary(BinOpKind::Shr)),
-            token::PlusEq => Some(AssignOp(BinOpKind::Add)),
-            token::MinusEq => Some(AssignOp(BinOpKind::Sub)),
-            token::StarEq => Some(AssignOp(BinOpKind::Mul)),
-            token::SlashEq => Some(AssignOp(BinOpKind::Div)),
-            token::PercentEq => Some(AssignOp(BinOpKind::Rem)),
-            token::CaretEq => Some(AssignOp(BinOpKind::BitXor)),
-            token::AndEq => Some(AssignOp(BinOpKind::BitAnd)),
-            token::OrEq => Some(AssignOp(BinOpKind::BitOr)),
-            token::ShlEq => Some(AssignOp(BinOpKind::Shl)),
-            token::ShrEq => Some(AssignOp(BinOpKind::Shr)),
+            token::PlusEq => Some(AssignOp(AssignOpKind::AddAssign)),
+            token::MinusEq => Some(AssignOp(AssignOpKind::SubAssign)),
+            token::StarEq => Some(AssignOp(AssignOpKind::MulAssign)),
+            token::SlashEq => Some(AssignOp(AssignOpKind::DivAssign)),
+            token::PercentEq => Some(AssignOp(AssignOpKind::RemAssign)),
+            token::CaretEq => Some(AssignOp(AssignOpKind::BitXorAssign)),
+            token::AndEq => Some(AssignOp(AssignOpKind::BitAndAssign)),
+            token::OrEq => Some(AssignOp(AssignOpKind::BitOrAssign)),
+            token::ShlEq => Some(AssignOp(AssignOpKind::ShlAssign)),
+            token::ShrEq => Some(AssignOp(AssignOpKind::ShrAssign)),
             token::Lt => Some(Binary(BinOpKind::Lt)),
             token::Le => Some(Binary(BinOpKind::Le)),
             token::Ge => Some(Binary(BinOpKind::Ge)),