Bug 1803750 - Fix build breakage for Early Beta sim regarding `refAsNonNull`...

Bug 1803750 - Fix build breakage for Early Beta sim regarding `refAsNonNull` in WasmIonCompile.cpp.  r=rhunt.

I believe bug 1799856 has caused this failure, by causing
FunctionCompiler::refAsNonNull (and its dependency ::compareIsNull) to be
guarded by ENABLE_WASM_FUNCTION_REFERENCES, whilst the uses are not similarly
guarded.

This patch makes those routines not be guarded by
ENABLE_WASM_FUNCTION_REFERENCES, so as to match the use points.

Differential Revision: https://phabricator.services.mozilla.com/D163734