Formalize MDNode's function-localness:

- an MDNode is designated as function-local when created, and continues to be even if its operands are modified not to refer to function-local IR
- function-localness is designated via lowest bit in SubclassData
- getLocalFunction() descends MDNode tree to see if it is consistently function-local

Add verification of MDNodes to checks that MDNodes are consistently function-local.
Update AsmWriter to use isFunctionLocal().

llvm-svn: 91708