* Update AIR instruction `intcast` to allow the dest type to be an
enum.
* LLVM backend: update `intcast` to support when the bit counts of
operand and dest type are the same. This was already a requirement of
the instruction previously.
* Type: `intInfo` supports the case when the type is an enum, and
retrieves the info for the integer tag type. This makes it pretty
easy for backends to implement `intcast` without having to care
explicitly that the new type is an enum. As a bonus, simple enums
never have to go through the type system; their signedness and bit
count are computed directly.
The "int to enum" behavior test case is now passing for stage2 in the
LLVM backend.
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