MIRParser: Use dot instead of colon to mark subregisters

Change the syntax to use `%0.sub8` to denote a subregister.

This seems like a more natural fit to denote subregisters; I also plan
to introduce a new ":classname" syntax in upcoming patches to denote the
register class of a vreg.

Note that this commit disallows plain identifiers to start with a '.'
character.  This shouldn't affect anything as external names/IR
references are all prefixed with '$'/'%', plain identifiers are only
used for instruction names, register mask names and subreg indexes.

Differential Revision: https://reviews.llvm.org/D22390

llvm-svn: 276815