[WebAssembly] Report error for inline assembly with unsupported opcodes

For inline WebAssembly, passing a numeric operand to global.get is
unsupported. This causes encodeInstruction to reach an llvm_unreachable
call, leading to undefined behaviors. This patch fixes the issue for
this invalid instruction encoding, making it report an error by adding
an MCContext field in class WebAssemblyMCCodeEmitter.

Reviewed By: sbc100, bryanpkc

Differential Revision: https://reviews.llvm.org/D154734