[SimplifyCFG] Don't merge invoke if this makes immarg non-constant (PR61265)

Don't merge invokes if this replaces constant operands with phis
in a place where this is not legal.

This also disallows converting operand bundles from constant to
non-constant, in line with the restriction we use in other
transforms.

Fixes https://github.com/llvm/llvm-project/issues/61265.

Differential Revision: https://reviews.llvm.org/D146723