MIRParser: Use shorter cfi identifiers

In an instruction like:
	CFI_INSTRUCTION .cfi_def_cfa ...
we can drop the '.cfi_' prefix since that should be obvious by the
context:
	CFI_INSTRUCTION def_cfa ...

While being a terser and cleaner syntax this also prepares to dropping
support for identifiers starting with a dot character so we can use it
for expressions.

Differential Revision: http://reviews.llvm.org/D22388

llvm-svn: 276785