AArch64: Don't force numbering of <dt> annotations in instruction pattern

[hanno-becker]

AArch64 instruction patterns cannot include multiple .<dt> specifiers. In this case, they must be numbered as .<dt0>, .<dt1>, ... This is unexpected, undocumented, and no useful error message is given when it happens.

• At the least, fail with an expressive error message if an instruction pattern includes more than one .<dt>
• Likely better, allow repeated .<dt> size specifiers, and force the respective specifiers to be equal during parsing

Noted by @nebeid