[WebAssembly] Fix unwind mismatch stat computation

Summary:
There was a bug when computing the number of unwind destination
mismatches in CFGStackify. When there are many mismatched calls that
share the same (original) destination BB, they have to be counted
separately.

This also fixes a typo and runs `fixUnwindMismatches` only when the wasm
exception handling is enabled. This is to prevent unnecessary
computations and does not change behavior.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68552

llvm-svn: 373975