[llvm-dev] [RFC] Should we add isa_or_null<>?

[Don Hinton via llvm-dev]

I'd like to propose adding `isa_or_null<>` to replace the following usage
pattern that's relatively common in conditionals:

  var && isa<T>(var)  =>>  isa_or_null<T>(var)

And in particular when `var` is a method call which might be expensive,
e.g.:

  X->foo() && isa<T>(X->foo())  =>>  isa_or_null<T>(X->foo())

The implementation could be a simple wrapper around isa<>, and while the IR
produced is only slightly more efficient, the elimination of an extra call
could be worthwhile.
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