[llvm] [GISel][AArch64] Allow PatLeafs to be imported in GISel which were previously causing warnings (PR #140935)

[Pierre van Houtryve via llvm-commits]

================
@@ -1293,8 +1293,21 @@ bool TreePredicateFn::hasGISelPredicateCode() const {
 }
 
 std::string TreePredicateFn::getGISelPredicateCode() const {
-  return std::string(
-      PatFragRec->getRecord()->getValueAsString("GISelPredicateCode"));
+  return PatFragRec->getRecord()->getValueAsString("GISelPredicateCode").str();
+}
+
+bool TreePredicateFn::hasGISelLeafPredicateCode() const {
+  return (!PatFragRec->getRecord()
+               ->getValueAsOptionalString("GISelLeafPredicateCode")
+               .value_or(std::string())
+               .empty());
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Pierre-vh wrote:

```suggestion
               .has_value()
```
or just use the `operator bool()`, `std::optional` evaluates to true/false depending on whether there is a value

https://github.com/llvm/llvm-project/pull/140935