RFC: Update token semantics with default convergent attribute

[sameerds]

Summary

This RFC has two parts:

LangRef:

Define the "convergent" property in LLVM IR and introduce the
"noconvergent" attribute. This is independent of convergence
control intrinsics and tokens.  This part is intended to be
submitted first and replaces D69498 (IR: Invert convergent
attribute handling)

ConvergentOperations:

Updates the semantics of convergence control intrinsics and tokens
to account for the new default convergent property. This part is
intended to be merged into D85603 (IR: Add convergence control
operand bundle and intrinsics)