[coro] Fix rematerializable instruction sinking to coro.suspend blocks

There is a constraint that coro.suspend instructions need to be in their
own blocks. The coro split pass initially creates IR that obeys this constraint
(which is later checked). Sinking rematerializable instructions into these
blocks breaks that constraint.

Instead rematerialize in the predecessor block to the suspend's single
predecessor block.

Differential Revision: https://reviews.llvm.org/D104051