PMI arm diffs issue

I saw this a few times in some arm altjit frameworks PMI diffs:
```
Base:

movw r3, 0xd1ff
movt r3, 0xd1ff
blx r3 // System.Threading.Tasks.Dataflow.BatchBlock`1[__Canon][System.__Canon]:get_DebuggerDisplayContent():ref:this

diff:

mov r3, 0x3d8000
blx r3 // System.Threading.Tasks.Dataflow.BatchBlock`1[__Canon][System.__Canon]:get_DebuggerDisplayContent():ref:this
```

I presume what’s happening is the addresses are different between base and diff, and we’re getting lucky to be able to encode a constant with one of arm’s wacky constant encodings. I don’t remember ever seeing this with ngen-base altjit asm diffs – presumably because the addresses always required relocs, so we always forced movw/movt encoding. This could be kind of annoying; we might have to thing about how to make this stable.

(This points out an advantage for SuperPMI diffs, where the addresses are guaranteed to be identical.)