[WebAssembly] Add tests for the Asm type checker

While looking at bugs like PR54022, I noted that there is no real test
coverage for the asm type checker. This patch starts to address that,
adding a series of tests for the errors messages produced, as well as
some FIXMEs as an XFAIL test for some current issues.

It's not intended to be an exhaustive test, but does have test cases for
each of the instructions that the type checker has specific handling
for.

Differential Revision: https://reviews.llvm.org/D122020