[WebAssembly] Use llvm utility functions in EH/SjLj

This uses `changeToCall` and `changeToInvokeAndSplitBasicBlock` from
`lib/Transforms/Utils`, replacing the custom logic. One difference of
those functions from our previous logic is they delete the original
`CallInst`/`InvokeInst`, which makes them tricky to use while iterating
through instructions/BBs. So this CL gathers the candidate calls first
and run them through `changeToInvokeAndSplitBasicBlock` later.

Also this renames some variables.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D116620