[llvm] [ARM][TableGen][MC] Change the ARM mnemonic operands to be optional for ASM parsing (PR #83436)

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

Also, for quite a nice example of improved behavior from this change, see `llvm/test/MC/ARM/tMOVSr.s`.

In this case, the old behavior was that the de-facto wide/narrow decision meant this was parsed as a wide encoding which was then converted to narrow by a hacky bit of code that this test checks for. 

This test is changed by this patch to check that the narrow encoding is used because it was selected by the TableGen matcher and the hack wasn't used (I intend to go through these hacks and remove all those that are now unused after this patch is accepted).

https://github.com/llvm/llvm-project/pull/83436