support new first-class denormal attributes

[regehr]

in Feb Matt redid FP denorm attributes so that they're real LLVM attrs instead of strings:
llvm/llvm-project@2502e3b7badc

so alive2 now has some false alarms due to failing to parse the new-style attributes: #1308

we don't want to remove the old string-based code path / tests yet since it looks like the text IR parser does not run the IR auto-upgrader. we can remove this code path / tests once LLVM drops support for the old string attribute syntax.

if @arsenm happens to have time to take a quick look at the test cases attached to this PR, that would be helpful

[arsenm]

once LLVM drops support for the old string attribute syntax.

It was dropped, it's only handled for bitcode autoupgrade