stage2: rework ZIR/TZIR for optionals and error unions

* fix wrong pointer const-ness when unwrapping optionals
 * allow grouped expressions and orelse as lvalues
 * ZIR for unwrapping optionals: no redundant deref
   - add notes to please don't use rlWrapPtr, this function should be
     deleted
 * catch and orelse: better ZIR for non-lvalue: no redundant deref;
   operate entirely on values. lvalue case still works properly.
   - properly propagate the result location into the target expression
 * Test harness: better output when tests fail due to compile errors.
 * TZIR: add instruction variants. These allow fewer TZIR instructions to
   be emitted from zir_sema. See the commit diff for per-instruction
   documentation.
   - is_null
   - is_non_null
   - is_null_ptr
   - is_non_null_ptr
   - is_err
   - is_err_ptr
   - optional_payload
   - optional_payload_ptr
 * TZIR: removed old naming convention instructions:
   - isnonnull
   - isnull
   - iserr
   - unwrap_optional
 * ZIR: add instruction variants. These allow fewer ZIR instructions to
   be emitted from astgen. See the commit diff for per-instruction
   documentation.
   - is_non_null
   - is_null
   - is_non_null_ptr
   - is_null_ptr
   - is_err
   - is_err_ptr
   - optional_payload_safe
   - optional_payload_unsafe
   - optional_payload_safe_ptr
   - optional_payload_unsafe_ptr
   - err_union_payload_safe
   - err_union_payload_unsafe
   - err_union_payload_safe_ptr
   - err_union_payload_unsafe_ptr
   - err_union_code
   - err_union_code_ptr
 * ZIR: removed old naming convention instructions:
   - isnonnull
   - isnull
   - iserr
   - unwrap_optional_safe
   - unwrap_optional_unsafe
   - unwrap_err_safe
   - unwrap_err_unsafe
   - unwrap_err_code

src/ir.zig

@@ -73,9 +73,18 @@ pub const Inst = struct {
         condbr,
         constant,
         dbg_stmt,
-        isnonnull,
-        isnull,
-        iserr,
+        // ?T => bool
+        is_null,
+        // ?T => bool (inverted logic)
+        is_non_null,
+        // *?T => bool
+        is_null_ptr,
+        // *?T => bool (inverted logic)
+        is_non_null_ptr,
+        // E!T => bool
+        is_err,
+        // *E!T => bool
+        is_err_ptr,
         booland,
         boolor,
         /// Read a value from a pointer.
@@ -93,7 +102,10 @@ pub const Inst = struct {
         not,
         floatcast,
         intcast,
-        unwrap_optional,
+        // ?T => T
+        optional_payload,
+        // *?T => *T
+        optional_payload_ptr,
         wrap_optional,
         xor,
         switchbr,
@@ -111,14 +123,18 @@ pub const Inst = struct {
                 .ret,
                 .bitcast,
                 .not,
-                .isnonnull,
-                .isnull,
-                .iserr,
+                .is_non_null,
+                .is_non_null_ptr,
+                .is_null,
+                .is_null_ptr,
+                .is_err,
+                .is_err_ptr,
                 .ptrtoint,
                 .floatcast,
                 .intcast,
                 .load,
-                .unwrap_optional,
+                .optional_payload,
+                .optional_payload_ptr,
                 .wrap_optional,
                 => UnOp,