[WebAssembly] Fixed AsmParser not allowing instructions with /

Summary:
The AsmParser Lexer regards these as a seperate token.
Here we expand the instruction name with them if they are
adjacent (no whitespace).

Tested: the basic-assembly.s test case has one case with a / in it.
The currently are also instructions with : in them, which we intend
to rename rather than fix them here.

Reviewers: tlively, dschuff

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

Differential Revision: https://reviews.llvm.org/D52442

llvm-svn: 343501