[Convergence] allow non-convergent ops before entry and loop intrinsics (#65939)

The only real requirement is that entry and loop intrinsics should not
be preceded by convergent operations in the same basic block. They do
not need to be the first in the block.

Relaxing the constraint on the entry and loop intrinsics avoids having
to make changes in the construction of LLVM IR, such as
getFirstInsertionPt(). It also avoids added complexity in the lowering
to Machine IR, where COPY instructions may be added to the start of the
basic block.