Avoid a bitcast FFI call in transmuting
For things that only change the valid ranges, we can just return the input, rather than making the `LLVMBuildBitCast` call and having *it* then do nothing.
I tried to tweak this a bit more and broke stuff, so I also added some extra tests for that as we apparently didn't have coverage.
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so: