[llvm] [MC][ARM] Fix crash when assembling Thumb 'movs r0, #foo'. (PR #115026)

[Eli Friedman via llvm-commits]

================
@@ -8653,6 +8657,41 @@ bool ARMAsmParser::validateInstruction(MCInst &Inst,
   return false;
 }
 
+// After processInstruction has transformed an instruction being assembled into
+// a different opcode, do any further validity checks that the new opcode
+// depends on.
+//
+// `Inst` contains the final modified form of the instruction, but `Operands`
+// contains the parsed operands from the _original_ instruction, because
+// nothing has updated them (`processInstruction` received them as const).
+// `OrigOpcode` contains the original value of `Inst.getOpcode()`, which should
+// give enough context to know how to understand the original operands list.
+bool ARMAsmParser::revalidateInstruction(MCInst &Inst,
+                                         const OperandVector &Operands,
+                                         unsigned MnemonicOpsEndInd,
+                                         unsigned OrigOpcode) {
+  const unsigned NewOpcode = Inst.getOpcode();
+
+  if (OrigOpcode == ARM::t2ADDri && NewOpcode == ARM::tADDi8) {
+    // t2ADDri is the Thumb 32-bit immediate add instruction, for example
+    // 'add[s] r0,r1,#0xff00'. If its immediate argument isn't a constant
+    // requiring shifting, then processInstruction can turn it into tADDi8, the
+    // simpler 16-bit Thumb immediate add (provided all the other conditions
+    // for that transformation are met). That makes it too late for
+    // validateInstruction to do this check, which it would have done if it had
+    // known from the start that the instruction was tADDi8.
----------------
efriedma-quic wrote:

Could the handling for t2ADDri just skip transforming the instruction if we know the end result isn't a valid Thumb1 instruction?  Seems weird to transform to a Thumb1 instruction we know won't work, then produce an error referencing the Thumb1 instruction instead of the original Thumb2 instruction.

Some additional testcases:

```
add r9, r0, #foo
movs r0, :lower16:#foo
movs r11, :upper8_15:#foo
```

https://github.com/llvm/llvm-project/pull/115026