[NFC][mlir][spirv] Fix syntax warnings in gen_spirv_dialect.py (#111775)
In the context of regular expressions, Python (used to) gracefully
ignore the escape behavior of `\` in some contexts, e.g. for
representing the regular expression `\w+`. However in newer versions of
Python this now gives a warning in the form
```
SyntaxWarning: invalid escape sequence '\w'
```
Fix by explicitly using raw strings instead.