Re-commit r363744: [tblgen][disasm] Allow multiple encodings to disassemble to the same instruction

It seems macOS lets you have ArrayRef<const X> even though this is apparently
forbidden by the language standard (Thanks MSVC++ for the clear error message).
Removed the problematic const's to fix this.

(It also seems I'm not receiving buildbot emails anymore and I'm trying to find
 out why. In the mean time I'll be polling lab.llvm.org to hopefully see if/when
 failures occur)

llvm-svn: 363753