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

[Alfie Richards via llvm-commits]

AlfieRichardsArm wrote:

If there is any interest from @llvm/issue-subscribers-backend-mips I noticed the sorting by smallest encoding has effects for the MIPS parsing (thus the inclusion of the option rather than having it on as default to not break tests). I don't know what the correct behavior is for MIPS.

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