Building with GCC produces:
```
<...>/TuneExtensionOps.cpp:180:26: warning: comparison of unsigned expression in ‘< 0’ is always false [-Wtype-limits]
180 | if (*selectedRegionIdx < 0 || *selectedRegionIdx >= getNumRegions())
| ~~~~~~~~~~~~~~~~~~~^~~
<...>/TuneExtensionOps.cpp: In member function ‘llvm::LogicalResult mlir::transform::tune::AlternativesOp::verify()’:
/home/david.spickett/llvm-project/mlir/lib/Dialect/Transform/TuneExtension/TuneExtensionOps.cpp:236:19: warning: comparison of unsigned expression in ‘< 0’ is always false [-Wtype-limits]
236 | if (regionIdx < 0 || regionIdx >= getNumRegions())
| ~~~~~~~~~~^~~
```
As we are sign extending these variables, use int64_t instead of size_t
for their type.
405403c8ed