[WebAssembly] Change label numbers to variables in test

cfg-stackify-eh.ll contains many `CHECK` lines specifying label / catch
comments with numbers. These numbers are subject to change every time
any block/loop/try is added in the middle in existing functions or any
other function is added in the middle of the file, generating a large
number of lines in diffs. This change converts them to variables so they
can be more resistent to future changes.

Reviewed By: dschuff, tlively

Differential Revision: https://reviews.llvm.org/D94037