[TableGen] Add IntrNoMerge as intrinsic property

There is a function attribute 'nomerge' in addition to 'noduplicate'
and 'convergent'. Both 'noduplicate' and 'convergent' have corresponding
intrinsic properties. This patch adds an intrinsic property for the
'nomerge' attribute.

Differential Revision: https://reviews.llvm.org/D96364