[WebAssembly] Remove unreachable EH pads

This removes unreachable EH pads in LateEHPrepare. This is not for
optimization but for preparation for CFGStackify. In CFGStackify, we
determine where to place `try` marker by computing the nearest common
dominator of all predecessors of an EH pad, but when an EH pad does not
have a predecessor, it becomes tricky. We can insert an empty dummy BB
before the EH pad and place the `try` there, but removing unreachable EH
pads is simpler.

This moves an existing exception label test from eh-label.mir to
exception.mir and adds a new test there.

This also adds some comments to existing methods.

Reviewed By: dschuff, tlively

Differential Revision: https://reviews.llvm.org/D94044